Yeah but I only got a 380 watt power supply.. I wonder if I can get by with that. 
As a 'what if' anyone got an opinion on the following CPU?
AMD Athlon 64 X2 5400+ Brisbane 2.8GHz 2 x 512KB L2 Cache Socket AM2 65W Dual-Core black edition Processor
I don't think the 380w PSU would work with the 3850. I suspect anything faster than a 6800 series card would need more power.
Nothing wrong with that cpu other than it's an AM2 socket processor and most (all?) Athlon 64 boards with AGP slots used socket 939 not AM2, and I thought most (all?) A64 3700s were socket 939 or 754.
If you're planning on buying a new mobo to go with a new cpu, then switch to intel this cycle.
If I'm not answering the question, then it's because you didn't really give any reason *why* you want an opinion on that cpu. It sounds like you're thinking about putting it into your current system, which seems like throwing good money after bad. And like I said, it would be a bit strange to be able to replace an A64 3700 on an AGP mobo with an AM2 cpu, since I thought AGP died before AM2 hit the market.
A google search for K8N tells me that that mobo is actually socket 754? Regardless, it uses an NForce3 chipset which pre-dated AM2 cpus.
If you want a better answer, you need to say why you want an opinion on that cpu
