My hopeful setup...

CPU: AMD Athlon 64 3000+ Venice
-How much of a performance boost would the 3700+ offer?
MOBO: DFI LANPARTY UT nF4 SLI-DR
-What else
GPU: MSI 7800gtx
-I play Battlefield 2 intensively...but might be easily persuaded to lower...
RAM: OCZ Gold Series 1GB (2 x 512MB) DDR 400 (PC 3200)
-Open to reasons for upping or lowering...

I'm planning on keeping my superalien case, but replacing the PSU with a Fortron 530w. Unless there is a nice...somewhat cheap...case. I actually prefer a plain look...

Do I really need a whole new heatsink/fan?
THERMALRIGHT XP-90 Heatsink
ZALMAN ALCU 92mm 2 Ball Cooling Fan

What would this sytem be capable of? I've never overclocked much except my GPU...

dfi is supposed to be confusing if you dont know what you're doing. might wanna look at epox. would you be planning on oc'ing the processor? if so stay with the 3000+. it should oc the same as the 3700+. fortron is pretty good. if you have the money i heard ocz is pretty good too. all in all i think this is a pretty good system.

I've heard about the difficulties of the DFI boards...I usually have no problems figuring things out with a little time and maybe research. Is it really that hard?

the difficulties probably come from people who havent read up on them, probably about compatability issues. ive been reading up on them and it doesnt seem too hard. (dont threadjack heh) wanna give him any advice with the board? id suggest reading up on dfi-street.com

Usually the DFI board is not recommended to people because it may take some tweaking in order to get it to work properly. However you can get help at places like DFI-Street and since you said yourself that you wouldn't mind doing taking some time to troubleshoot I think it would be a fine choice. The DFI is the best overclocker available. Do you really need SLI though? I highly doubt you do and it would be a wiser choice to just get the non-SLI version of that board. Then the money can be used towards other things that would be more beneficial.

Also definitely upgrade to 2GB of RAM. You specifically mentioned BF2 and you would benefit a lot more from cheap 2GB of RAM as opposed to 1GB of "performance" RAM. Just get some cheap crucial sticks. They overclock really well for a reasonable price. I wouldn't suggest spending over $215 for memory. Anything over would have been money well spent in other places.

I personally would just get the 7800GT. The 7800GTx seems to be a waste of money to me for $100 extra. If you don't plan on playing your games at least at 1600x1200 I wouldn't even bother with a 7800 though. A 6800GT would be fine.

As for the PSI I suggest the Fortron AX500-A. I also suggest just sticking with the 3000+. If I were to go with anything more expensive it would be dual-core.

Whether or not you need extra cooling is up to you. If you just plan to do some light overclocking the stock should be fine since Venice cores are so cool. However if you are only planning on a light overclock you should look at a different motherboard, such as the Epox.

Depends on more or less if your OCing.

If you want to OC, get the DFI of course, and the 7800GT would be a fine choice.

But if your only planning on doing mild OC, maybe the 3700+ would be a better alternative, and the 7800GTX would have a performance increase.

I also recommend 2GB of ram for BF2, many say, and some reviews have said, that 2GB can increase your FPS by as much as 10-15. If your planning to OC, get the OCZ Platinum 2GB set, it has better timings and easier to OC. But if you don't need to OC, 2GB Value Ram should do fine.

If your willing to spend time and have patience, the DFI is a better choice for extreme OCing. It'll give you more mhz then compared to an Epox or MSI board. But if you only want to OC a little bit, the Epox board is a great choice.

Only get a new heatsink if you want to OC above 100mhz, some can get 200mhz, but it's usually a tad in the danger spot. I'd probably get the XP-90C, I like it better then any Zalmans.

The Forton AX500-A is a great budget choice PSU. OCZ would be the better alternative, but it's about $20 more then the Forton.

It all depends on your budget and what you'll be doing with it. If your budget can afford it, the 7800GTX is a much better choice with the extra pipelines and such.