The Athlon 64 3200+ is better then the Athlon XP 2000+.
http://www.techpowerup.com/cpudb/Your graphics card is fine. Its not powerfull but it is relatively new, or at least in the current generation of cards. CPUs dont tend to be bottlenecked as much as GPUs, so that issue is pretty much solved anyway.
Before i get to the problem, i have to ask though.. why do u have 2.5 GB of RAM? There is no game on Earth that would see a performance difference with that. Its a complete waste of money and power. So u spent have the worst graphics card of this current generation and yet u have enough RAM to power a high-end game from 2009?
Ok, so now ill tell u what i think the problem is.
Its ur PSU.
The recomended specs for ur card include a 400w PSU, and thats the minimum. You have an modern CPU and enough RAM to power the space shuttle, and who knows what else ur powering..
But, i still dont u know what u mean by "losing performance", do u mean that the performance was greater before, maybe when u have the 2000+ and the 9550? If so, then its definetely ur PSU.
Remmember to get a brand-name PSU. A bran-name 350w for instence would be better then an unknown brand 400w PSU, and also as the only thing keeping all ur hardware from dying, its best to get one that will last.