Vista is fine if you want to upgrade, XP is fine if you don't. You're solid either way in my opinion, I'm too cheap/broke to buy a copy of Vista to upgrade to so I just stay with XP, no problems here. Corsair is a great company to get ram from, they have that $25 4gb set after rebate, good deal as well.
As for the motherboards, it depends on what you need. The P45s are great for overclockers, still good even if you don't overclock. If you don't overclock or have some special need for Crossfire or SLI, any mother that supports your chip and is from a reputable company with a decent warranty would be fine, whether it cost you $50 or $300, it won't make too much of a difference (except maybe the onboard audio, if you care for the differences between the chips).
On to the video card, also based on what you need/intend to use it for. The games you've mentioned aren't exactly graphically intensive and therefore don't need a powerful GPU to give you great visuals and higher than most monitors can support for frame/refresh rates. And the 9800 GT is on par with or better than the 4830, while lower than the 4850. Any one of those would be a good choice depending on your monitor resolution.
For the processor, most games today will benefit the most from a high clocked dual core over a quad (unless similarly clocked of course) since most games only support up to (if even) 2 cores, 4 cores are only necessary for games with loads of AI or physics that get offloaded to the CPU for whatever reason. Big RTS games like SupCom would require a quad core for large games, and apparently so does GTA4 (exception for now, though developers seem to be making the move towards quad-optimized). The Q9400 is a good quad, but I can't help but feel you'd be fine with an E8400 (unless you plan on playing GTA4).