Go with XP. For the following reasons:

• Vista is a resource hog (nearly a half gig of your memory goes to just supporting the vista OS!).
• Vista has extensive DRM in the very core of the OS. These DRM restrictions require computational over-head and also model 'the user' in a way that is, IMHO, extremely questionable.
• There are few or no security enhancements to the Vista OS over the XP OS, which was the central problem with XP. In fact, it looks like to me that Vista is worse than XP was WRT security.
• Vista is too young to really know if it has critical stability problems- or worse, if it has critical unsolvable security flaws (such as 95 had, for example)
• if you know how to run XP already, and all your software works with XP (or 2000... XP and 2000 are virtually the same bird), why bother switching? XP will probably be supported by MS for 3-4 years given the market penetration of XP and judging by the lifecycle of 2000. Unless you are 'really into' games, then there is no real need for a vista system.

WRT processors:

• AMD usually has better 'bang for the buck'. Intel usually holds the speed title for the top-line processors (though this is not always true, AMD is catching up). AMD desktop chips historically tend to run cooler (though this is not historically true for laptop chips). Again, there have been notable exceptions to this, but typically $300 of AMD out-performs $300 of Intel. AMDs are also very durable, with significant 'reserves'- making them the usually the favorites of the over-clocking crowd
• Unless you are doing software development or runing server services, or CAD/graphics, 64-bit is more trouble than its worth for the average user. Run a 32 bit OS
• Last I checked, most 'moderate to high' performance processors were dual core. However, it is unlikely that you will see much advantage to multicore if you are an average user- IE, surfing the net, playing a few basic games, and running MS office products

Consider making the Mac OS jump. Mac machines have over-stated performance parameters, but they tend to be durable quality machines. Mac OSX is built on top of Unix, so you can make use of the Shell to start learning how to navigate and operae using the 'nix family. Making it easier to eventually transfer to a 'nix machine.

For my usage constraints (mainly software development), I find the main needs are for maximal memory and maximal hard drive speed and space. Consider a multi-disk software raid solution if disk access is your primary use bottleneck (most non-game entertainment fits this bill).

Consider also building your own box instead of buying a pre-made system. It used to be that building your own resulted in a cheaper system, performance-for-dollar wise. However, that is less true today. What you mainly get today by building your own is more quality/durability for the dollar. Also more upgradeability. Manufacturers like Dell cut corners on component quality, and also do stupid little things to make sure that their upgrades work the best on their systems. Building your own allows you to choose the best individual components for your money. It also gives you all the skills you need to upgrade and maintain your own system.

If you build your own, make the Box your main concern. Get a rail-mount solution with a good burnished-metal heavy case. Get something with a lot of room. Open it up and look inside and make sure there there are little cheap-plastic parts or razor sharp edges. Make sure the box is roomy and has all the fasteners you need. Make sure it has ports for additional cooling fans and use them. a cool system is a durable and long-lived system.

A good Box will serve as the platform for several successive systems, so amortize the cost over your next two systems. Same with the powersupply.

Here is my memory WRT components from my last build 3 years ago- so its probably seriously out of date.

Memory and HD: Hit/Miss commodity item with little reason to buy by brand. You tend to get what you pay for, though, so avoid super-cheap bits. Invest in a raid array, or a single High speed high cap SCSI drive. Get disks that you would find in Servers! SCSI drives are typically more durable and have higher spindle speeds available, with more cache memory. IDE drives have really improved, but SCSI (last I checked) is still the gold standard.

Motherboard: Very important. Asus and MSI used to be the best. Go to places like Toms Hardware and read up on reviews. Look for an mobo that is durable and easy to work with, and has all the slots and features you need. Look for on-board raid controllers.

Video-Card: IMHO, after HD, these are the most likely to fail. I have no advice, as the market for video cards was changing every month when I built my last system.

Power-Supply: VERY IMPORTANT. Get a power supply that has A LOT more muscle than you need. Higher power power supplies are usually higher-quality power supplies. More over-head allows you room to improve and add-on to your system.

A lot of people think they are giving up warranty when they build their own. Outside of installation error, thats not really true. Find a distributor who has the products you want, and work with them exclusiely. Find a medium-sized guy and get to know someone in the company. I've had excellent response to failed components.

I have pretty much become a laptop fan, so I probably will never build another system (as laptops are pretty much impossible to roll-your-own)

Keep your system cool, and upgrade rather than over-clock, and your system will have a long lifespan. You can find all the illustrated tutorials you need online to do whatever you need.

Yeti