best that unit can do without mods is:
Athlon 64x2 5600+ (2.9ghz) Brisbane 65nm processor
some Windsor class processors will work but those are more power hungry, more heat and older technology.
So according to the link posts you post the +5600 would be ideally better rather than the +6000 which would be too much for the C521( as you said). But are you really sure the +6000 won't work? I know you said it wont LOL but I just want to the best for the PC if you understand. Its not that i doubt you. Thanks again
Oh yea i forgot. I don't know your giving me this advice on the basis of my PC having a 280watt PSU. I have a Omega 600Watt ATX PSU running externally. So, that being said am i still limited to the +5600 which i think is still weak. I use this PC primarily for Flight Simulator 2004 and near future FSX. I also have a 9600Gt card.