P4 2.4Ghz - 256Mb RAM - GeForce 4 Ti4200 128Mb RAM - AGP4X and games are slow !!!

Hi everyone.
I have a very serious problem here.
I'm running WinXP with the configuration described in the subject of this
post and I have tried playing games like :
Warcraft III (starts, and then it goes like 1 frame every five seconds),
Dungeon Siege (hangs after 20 seconds),
Operation Flashpoint (hangs after 5 seconds)

I don't know what's wrong and I don't have the slightest idea of where to
start to fix my problems. I don't know what the problem might be. I have
downloaded the latest driver from Nvidia and I still got problems.
I have read that sometimes power supply issues could be the source of this
kind of issues, like say there's not enough power to supply the video card.
But that seems so unlikely, I mean, I have a 400 W power supply, that should
make it... shouldn'it ?

But then, I don't know what other settings I should check (configuration in
the BIOS ? in the control pannel of my video card ? which options to check ?
RAM managing options ? IRQ conflicts with sound card ?... please could
someone give me hints to solve my problem ? I really don't know where to
start to diagnos what's wrong and I know this card is able to render
something more than just StarCraft and Age of Empires.. argh !

Any help would be very much appreciated. Thanxs

Diego