Motherboard:
Why did you pick such an expensive SLI motherboard? Are you planning on using SLI? If not, (and I recommend not on your budget) I would drop that pricey motherboard and get something along the lines of
Epox EP-9NPA3Ultra for $85 (has onboard sound, gbit Lan, raid and sata 3gb/s).
CPUs....you need to decide if you want to Dual or Single core and if you want to overclock and how much of an overclock.
Athlon/ Opteron:
As far as the opterons, they are hand picked chips that AMD planned to sell for server applications so they tend to be good at overclocking, however if you aren't planning on an aggressive overclock, then the AMD Athlon 64 is fine if it saves you some money.
Single/Dual Core:
Opterons and Athlons both come in Dual or single core versions, your choice here really will depend on if you plan to multitask now or in the future and how long you need to keep this current cpu you will buy....most applications/games currently available (oblivion is one exception) don't get a huge benefit from dual cores (unless you are running other tasks concurrently). But of course, future applications will, or if you plan to do things in the background while gaming dual cores will be more important.
Are you planning to overclock? Chances are you can get a lot more umph out of these chips will a little overclock effort....but if you can't be bothered, you may want to pay more upfront for higher MHZ. See the overclocking forums for more info here.
One strategy would be to buy a single core 3700 or opteron 148 for now .... in a few months overclock it as much as you are comfortable and then in a year or so, upgrade to a dual core which theoretically will be cheaper then so you could get a faster dual core later for the price of a 3800 now...of course, this strategy relies on you being able to sell your old cpu on ebay. This would keep some money in your pocket now to buy other things and you probably wouldnt miss the dual core for now anyway.
Athlon 64 3700 2.2 ghz single core- $235 (should be fine if you dont need the MAX overclock potential)
opteron 148 2.2 ghz sinlge core - $265 (worth the extra $30 if you are planning on a big overclock)
opteron 165 2.0 ghz
Dual core - $327 (worth the extra $30 if you are planning on a big overclock)
Amd 3800 2.0 ghz
Dual Core - $297
Epox Motherboard $85
7900GT ~ $300 (recommended brands, evga, bfg for warranties and evga has a 90 trade up program)
So, by picking a less pricey motherboard, you should be able to get almost any processor on that list and the 7900gt as well.
FYI, if you didn't know, AMD is launching their AM2 Platform soon which will use DDR2 ram.....and Intel is releasing Conroe in the fall that looks to be very fast! (but I wouldn't let that stop you from upgrading now)