Spend the extra and get a 90nm socket 939 Athlon64. At this point buying a regular athlon XP or sempron cpu is pretty much just investing in old technology, and a good way to waste money. The athlon64 is only $60-100ish more than a Barton cpu and the motherboards aren't overpriced either. Check
http://www.newegg.com. I like the msi K8N neo2 platinum motherboard (nvidia based) for only $110, and either the athlon64 3000+ for $149 or the 3200+ for $185. Add on a gig of either nice OCZ memory (expensive) or regular crucial.com memory (high quality but not uber expensive) for 2x $89.99 (2 sticks of 512 pc3200 DDR) and you're in business.
$110
$185
$180
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$475
Toss in a thermalright heatsink and you're set, or spend the extra $20 and get the retail cpu instead of the OEM one and just use the mediocre AMD supplied heatsink and hope your case has enough airflow. The all copper thermalright heatsinks are the best air cooling solutions I've ever seen without exception, but they are very heavy. The thermalright aluminum ones also use heatpipes and cool almost as well as the copper ones but they weigh about 1/3 the copper ones so they're more hardware friendly. They all allow you to pick any type or size of fan you like, so you can get a good speed adjustable fan and turn the fan down for web browsing and up for gaming to keep the noise under control during normal use.
Don't forget a good power supply. I recently got a thermaltake silent purepower 480 for $55 at newegg. It has a fan speed adjustment knob that can go either in a front plate or backplate, and it's enough power for most mainstream gaming rigs. You don't really need 500+ watts unless you plan on adding in several hard drives plus one of the newest vid cards plus overclocking your cpu.
I just dropped $400 on an Nvidia 6800GT and it overclocks up to 6800 Ultra speeds pretty easily for $200 less. It's a bit overkill for AH2 but it would be a great match for an athlon64 gaming rig. Alternately, consider a regular nvidia 6800 or 6600GT for somewhere between $200 and $250.
That's what I'd get, and this is pretty much the parts list I've been considering for my own rig. I'd probably get the more expensive OCZ memory to allow for some overclocking overhead on the memory speed. OCZ platinum memory is nice and extremely fast, but it costs almost twice what the normal or value stuff costs.
