For those that are building a new gaming machine, I will recommend the best 2 scenarios according to what kind of gamer you are and how deep your pockets are.
Scenario 1 - You'll only settle for the high resolution + max graphics on a widescreen monitor (AND you have deep pockets).
Recommendation:
Get yourself a dual boot Windows XP 32 bit and Windows Vista 64 bit. XP will allow you to have a fallback in case vista doesn't work out for you. Vista on the other hand will provide you with some eye candies especially with titles such as Crysis and many other titles coming up in 2008.
Reasoning:
Windows Vista 64 bit will take full advantage of the new Quad Processors and therefore will process threads much faster for an overall good performance value. Upcoming big titles (2008 titles) will take advantage of multicore processing...especially when calculating advanced physics and AI in a game. However, when purchasing your components such as sound card, network card (or adapter) and so on, you will have to not only look at price and performance but COMPATIBILITY. This would be my biggest issue with Vista-64. Today, many components don't run at their full potential because their provider did not write Vista 64 bit drivers and in some cases, the components are simply not compatible with Vista-64, or are forced to be run under the 32-bit simulator in Vista. The simulator DOES slow performance a lot which explains why people find their PC soooo sloooow in Vista-64…but is semi-compatible I guess.
Windows XP 32 bit is the best Gamers OS today. Stable, makes your last generation PC look fast and all drivers and software compatible. Nonetheless, it’s the OS to fall back to should you have issues with 64-bit of Vista.
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Scenario 2 - You wish you had 2000$+ to spend on a PC for the next generation but you don't (Using 17" or 19" monitor).
Recommendation:
Stick with Windows XP 32-bit and us it as your main OS. If you dual boot then get Windows Vista 32-bit as it is indeed a pretty good OS and is somewhat stable for new components. Keep in mind that I am not referring to older components because that would be a whole new story!
Reasoning:
XP will most likely be your main OS and the software developers do come up with some pretty good graphics under DX 9 compatible games (which is pretty much every game until very late next year or more). On the other hand, Vista 32-bit will take good advantage of your Dual Core CPU and is more “driver support friendly” than his big brother, Vista 64-bit.
Scenario 1 - Hardcore PC
Nota 1: When building your machine, check and make sure that every component you are buying has drivers available in Vista 64-bit usually found on the producer's official support website. For instance, I had a pretty hard time finding a simple USB Wiresless Network adapter that held Vista-64 bit drivers. I, on the other hand, don’t have any networking issues like many I have seen in this forum.
Nota 2: Vista Ready is only a marketing scam and does not necessarily mean that drivers are available in Vista 64-bit. So make sure you have downloadable drivers FOR 64-bit.
Scenario 2 - High-End PC
Nota 1: If you use XP-32 bit as your main OS, don't waste money on even the cheapest Quad CPU as it will not use it even near to its potential. Buy a much better dual core and you will get equal to better results.
Nota 2: General rule: AMD CPUs and ATI Videocards are cheaper but will provide less performance than the equivalent in Intel CPUs and Nvidia Videocards which are much more expensive. I recommend comparing products with Tomshardare benchmarks and similar sites to find the best price-performance value according to your budget.
Finally, if you have the cash to burn then go big. Who wants to build a gaming PC that would only last a year? Well many of us don't play the Crysis games and will settle with less power hungry games such as HL2. So it all comes down to what kind of gamer you are (or will be for the next year).
Hope this helps!