Wi fi is a deal breaker? You know you the Gigabyte is cheaper and you could just get yourself a much better Wi Fi card or Wi Fi usb card with the money saved. The Wifi with the Asus is probably a cheap unit, I would rather get something from a better brand . That's just a suggestion though.
If you're gaming and have the extra money, get the GTS 640MB you wont be sorry and it'll last longer before loosing performance in newer games.
PSU the Corsair 620W would be better than the SilverStone 650W I think. Silverstone makes great psu's, but the Corsair is better in my opinion.
As for vista... well... all I can say is I've had Vista 64 since it came out and I have had absolutely zero problems with the OS. Some of the headaches were from manufactures that failed to bring out vista/vista 64 drivers, but that's not MS's fault. Now, however there are plenty of drivers for almost all programs to run them on 64bit.
Good luck with your build though . If you have questions msg me, AIM/Yahoo Maximus9102.
Not going to nit pick alternatives, of course there are some. Looks like a solid build.
I think the 320 is a good choice. By the time you need more, chances are better, faster, cheaper DX10 cards will be out there.
If you prefer Gigabyte, you can get PCI wireless. Should it ever fail, you won't need to replace the entire mobo (or buy a wireless card).
Has more bang for the buck then Creative Sound Blaster X-Fi XtremeGamer 7.1 and less bloatware. Plus it comes with TOSLINK Fiber Optic Digital Cable, if you want to connect to HDTV or Home Theater system.
I built a computer with a 8800GTS 320MB in an Apevia X-Cruiser with room to spare, so you are fine in the P180. I think that the P180 also has re-configurable hard drive bays so you should have no problem. That computer also had Vista Home Premium and the only issue was a game not working (FPS Creator). Otherwise, it's a very nice OS in terms of looks, functionality, and ease of use. As the others said, onboard Wi-Fi doesn't have to be a deal breaker, PCI cards are the cheapest and you can extend their range with hi-gain antennas if you need to (the onboard Asus wi-fi may or may not have a standard connection). The rest of your computer sounds solid, good luck!
Just to add to what has been said:
PSU: Corsair's are really great; quality wise and they're quiet! Got the best reviews at silentpcforum as well (it's important to me since I've lived with a quality Sparkle Power for the past 5 years, but I'm going deaf at this point). Not to add to the confusion, but the 520HX will be enough for a few future upgrades as well.
As Maziar said, get the OC'ed 320 if you game under 1600x1200, otherwise its not worth it. Better save the $100 and upgrade a bit sooner.
I hear to stay away from DDR2 and DDR3 combo motherboards, but since I can't back that up I'd just wait for someone else to explain it better.
EDIT: Meh, you beat me to the reply button regarding the Mobo issue