I dont agree with some of hte posters here.
I have a d805 running at 3.4Ghz with a Golden Orb 2 for cooling. I built the system almost half a year ago, so the core2 was not yet available - at the time it was the best price/performance option. If needed, I can run it as high as 3.8Ghz - and thats from a <100$ processor.
Whereas an athlon alternative is also a good choice, and is fairly cheap to assemble now - your options for upgrade are limited. The d805 stock performance is rather laughable, but overclock it to a 166FSB (or higher if you have a decent fan) and you performance on par with the top model in that line (d840) or higher. Pairing the d805 with a p965 based chipset mobo will give you a decent system NOW (provided you overclock), as well as the option to upgrade to Core2 when you feel your computer is just too slow.
Right now I'm running a
d805@ 3.4Ghz with an Nforce4 based mobo (Ni8 SLI)
2*512mb Corsair XMS2 (synchronous with fsb)
evga 7900GTO (stock speed for now)
I can tell you honestly that there is no game (except of course, Oblivion) where the performance of my PC does not satisfy me. Sure, if I had an E6400 running at 3 Ghz instead of my d805, I might have had 83FPS while playing FEAR instead of 66.
Currently, very few games are more CPU than GPU dependent, so your video card will always be the one holding oyu back. In fact, I first bought an EVGA 7600GT, thinking it's a decent card - and it was ! But sadly even with my d805 my video card was the one holding me back in games like FEAR or Company of Heroes.
I am not about to recommend a dxxx series dual core over a C2D, but I can tell you with all honesty that a computer with an x6800 and a 7600gs will be a much worse gaming machine than a computer with a pentium 4 3Ghz and a 7900GT. It's true, some games are more CPU intensive, but the GPU is always the more important factor.
The bottom line, in my opinion, is this. Get yourself the best video card you can afford with your budget (the x1950pro or the 7900GS can both be found for well under 200$ nowadays, and offer very decent performance; also look out for 1900xt's and 7900GTO on sale, but those are getting rare). Then find yourself a motherboard supporting Core2Duo (I suggest one based on the p965, as it has proved to be a good overclocker - Gigabyte or Asus, and possibly Foxconn are all reasonable options). If you're low on cash when you get to the CPU choice, your performance will not suffer significantly if you opt for an overlocked d805 instead of a core2. You really need a monster of a graphics card to take advantage of all the processing power offered by a high end core2, and since you're probably not in the market for the likes of a 8800GTX just yet, I think you can get away with a cheaper CPU for now.
Just my 2c, hope it helps
CHeers