Barnetti,
You seem to have a good build there. Seems like a very good price too. That would currently be considered a good mid-high end unit for gaming. You chose a good motherboard which will suit you for some time too.
The component that will hold you back....always....is the video card. But you have a board that supports PCI-Express, so in the future you'll just add more memory and a better video card
The video card you chose right now is by far the best bang per buck you're going to get. There are plenty of reviews out there showing it out performing the 512MB Radeon HD2600 (which is the ATI equivalent priced card). It has a good core/memory clock rate and supports DX10.
400 watt is a bit borderline for that build. But it should be enough. Basically, if you have a good surge protector (less than a few years old that has a good joule rating) then I would just try the power supply that comes with the case. If you notice the frame rate in games is overly sluggish and you know your machine can do much better, it's usually cause of low power. I would then step it up to a good 500w Power Supply Unit.
The only other thing I'd be considering is the brand of video card and memory you have. Try to hit up some other sites, or look for more products on that site to see how their reviews are. If you see lots of bad reviews for more of their products, you might want to look into different brands. You should be able to find better brands for very near those prices.
Definitely stick with XP. Vista is such a hardware hog. DX10 isn't highly utilized yet, so I wouldn't worry about it for now.
Right now I'm running the same
CPU, a 6600 Geforce card, and 3 gigs of PC3200 and I run Tiberium Wars with almost all high settings @ 1280 x 1024 very well. I just have to lower some more hardware dependent settings like clouds and terrain detail a little bit. Your build is a ton better than mine.
EDIT - I just noticed that you have a regular ATA hard drive. Change that to a SATA 3GB /sec hard drive with as low latency as you're willing to spend. You really shouldn't have trouble finding one near the same price as the ATA.
