k, there are charts that show the different frequencies that'll work best with ddr2 800 on the x2 procs... so mem, may or may not be a good choice, look it up on the articles on toms.... and an 8600? may or may not be a bad choice... depending... stream procs are good... on the 8 series... performance vs. price on 7900gs is awesome now... hd 2600's just released for dirt cheap... keep an eye on prices and what not.
1. proc speed on an amd x2 (due to integrated mem controller (intell will switch to as well) cirtain proc speeds will run your ram at full speed... so watch that and see what your best results will be... eg. if your current proc will run ddr2 800 at full speed... upgrade your memory... if it won't and the new proc will... upgrade the proc, then upgrade the ram later.
2. as for the 8600... depends on gt or gts. gt is not worth what it costs as far as i'm concerned and as far as most (legitimate) reviews are as well. the gts is a better buy. waiting a few weeks/months? wait to see what ati hits with the 2600's... the price is hot and the ACTUAL performance has yet to be seen.
3. PSU... I don't think the new hd 2600's require an external power connector (look it up i'm not sure) but i know that most video cards these days are going to suck down a butt load of power... 12 amps isn't enough... i'm running 16 amps on my +12v and i'm very limited... so the new ati's may be a good choice... just make sure you check the amp requirements on any video card you may or may not buy... especially if you like pretty psu's (cuz you can get cheap 36amp psu's but they're ugly and loud
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errr... no other thoughts now.... pm me with questions or whatever. I'll give ya an honest answer to the best of my knowledge ... just do like santa... check your list twice before you buy.