What will you be using the computer for? I judge by your card choice that you will be gaming, but is there anything else? How long do you plan on keeping it? Are you planning on SLI in the future? Do you plan on overclocking? What's your budget?
Now note, that many of my suggestions are usually aimed at saving money. I see you've picked out many high end parts (overly expensive), but if you're really set on it, can afford it, and want your computer to last as long as possible, then that's fine.
First of all, I can see you've somewhat set yourself up for some SLI possibilites. The 9800GTX is already a beastly card. I'm not sure what settings you're planning on gaming at, but SLI'ing two or more 9800GTXs would be overkill. If you're dead set on SLI, you could do two
8800GTS instead. It would be cheaper than 2 9800GTXs, and would offer nearly identical perfomance. If you're sticking to one card though, you could go with the
8800GTX or the
9800GX2 if you want to get frisky. Good choice picking EVGA though, they offer nice warranties and the 90 day step up service. They also make solid cards.
As far as power goes, 700-800w is about what you're looking at. I've never heard anything about BFG making PSUs, and I see the one you picked out doesn't have any reviews at all. Not only that, but it's a bit on the expensive side. I would stay away from it if possible, although I do like that it has decent amps on the rails, and it has a lifetime warranty. This
Corsair should meet your needs, unless you're planning on Tri-SLI, which I doubt (that would be reeeeaaaally overkill). Plus, it has a nice sal price at the moment.
Time for the motherboard and RAM. Again, unless you really plan on SLI, I would get a cheaper board. DD3 is really expensive right now, and the boost it gives over DDR2 is minimal. Plus, DDR2 is cheap right now. If you want to go a cheaper route, this
Gigabyte board and this
OCZ or this
Corsair should do quite nicely. This still leaves room for SLI as well, just not Tri-SLI. The Corsair RAM is more expensive, but it comes with a free RAM cooler, which is neat I guess. The motherboard is meant to be durable, plus I can see it supports the 45nm processors.
Now for the processor. If you only plan on playing games, then a quad won't do you too much good at the moment. However, if you plan on doing some Photoshop, video editing, and other such taxing activities, or if you just want your computer to be future proof for as long as possible, then your Q9300 is a great choice. If you want a dual core however, the
E8400 is also a nice choice. Either way, if you plan on overclocking, I'd recommend something other than the stock heatsink. This
Xigmatek is a great cooler, and it's cheap to boot. I can't comment on the sound card, since I don't use them. The only one I've ever used is a cheap $20 one for my old computer when the onboard sound died.
You have some mega storage going on there. The price is nice, plus they have 32MB cache. Very good. The OS is always a personal choice, so I won't comment on that either. Although I will say that Vista Ultimate is good for futureproofing, since I think it comes with both a 32-bit and 64-bit installation disk. The case is also a personal favorite of mine. The only thing you need to realize for that case is that the hard drive cooling is less than optimal. Your drives aren't Raptors or anything, so I'm sure they'll be fine though. They fixed that problem in the future revision of the Cosmos, but it's so ugly, that I couldn't justify purchasing it. The other case I like right now is the
Antec 1200. It has some nice cooling like it's predecessor, but is larger, and has included cable management holes and such behind the motherboard tray. The only thing I don't like about it is the side panel window. I just don't like side windows. Also, there's the gaudy blue lights it comes with, but you can replace the fans easily.
So, yeah until I know your plans, that's all I can give you.
Athlon X2 5000 BE 3.1GHz, Arctic Cooling Freezer 64 Pro, Gigabyte GA-MA770-DS3 AM2+, Visiontek HD4850, Corsair XMS2 2GB DDR2 800, OCZ StealthXStream 600W PSU