I currently have an Antec True550P and I suspect it might be the cause of the performance issues I am having.
The following is the original spec of my rig when I first got this PSU:
Athlon 3500+ Newcastle
Hitachi Deskstar 7200 Sata x 2 (Raid)
4 x 512mb Corsair C2 Platinum
Asus A8V Deluxe mobo
Antec True550P (550 watt)
XP Pro 64 bit
Performance was very satisfactory and I had no quarms with my system. Since, I have made the following changes to my system:
X2 4400 Toledo
Sapphire 1950 Pro 512mb
WD Raptor 36gb x 2 (Raid0) + Hitachi Deskstar 7200 Sata x 2 (Raid) making 4 drives in total
Performance, woeful to say the least. I noticed the difference straight-away, things were not as smooth and most noticeable was the stuttering that occured when panning in game (joint ops). I cannot pan 180 degrees in game without it stuttering. This stutter would be momentary but enough to put me at a disadvantage when in a shoot-out. Fraps show 40+ fps constantly but it does not feel sharp nor responsive.
FEAR just murders my system after 5 minutes, it starts to play like a slideshow.
I do not think that temp is the problem as I am showing no more than 57 Celius on my card when playing.
The only thing I can think is that my current spec is just too demanding on the PSU, especially the 12Volt rail and i am currently thinking of buying this PSU as the last effort to solve this problem before I swap my 1950 pro for a 6800GT!
Did you uninstall and remove nvidia driver with cleaner before installing ATI driver? PSU usually causes instability or crashing/rebooting. Ditch the 64bit XP, there is no advantage, and plenty of disadvantages. Have you applied AMD patch for x2 CPUs? Have you checked if XP 64Pro requires Micro$oft patch for dual core, there was a patch for 32bit. Did you get full set of 64bit drivers for ALL your hardware, especially mptherboard?
I have an X1950 XT and and I run my C2D at 3+Ghz and fear tends to kill my FPS also, so its no suprise to me that your 1950 pro is having issues since its slower than my card plus you have a slower CPU.
Another quick thing to check is that you may have a game that just doesn't like a dual core. I don't have many games here but one certainly (Tron 2, I think quake 3 engine) works much better when instructed to use one core. Easy enough to try, ctrl-alt-dlt to bring up the control thingy, find the prog and set the affinity to the first cpu.
Doesn't explain your general sluggishness though if you have all the drivers up to date along with the xp/amd patches.
I would think that you have enough juice in the power supply and the ATI card only takes a few more watts than the 6800GT it replaces. Only thing along those lines that might be worth a quick investigation is seeing how the cables are distributed out, if the Antec has dual 12V rails it'll split that 30A between them, might be worth making sure the graphics card isn't sharing too much.
LOL and of course check your DMA is enabled on IDE devices...can't believe I got caught out with that again yesterday after reloading my system!
Oh well, none of this may be of use, but you never know. Hope you get it sorted.