Question about Graphics card/Power supply..

Hello everyone, I recently purchased a GeForce GTX550Ti 1gig DDR5 (it was on sale) to replace my 9500 GT 512MB DDR3. When I read the description it said a minimum of 400watt psu, which I have, but I didn't see anything about the 12 volt rail needing 24 amps and didn't even notice until after the install. My psu only gives 20 amps and when I installed the card there was no change in performance. So my questions is, could not having the 24 amps be whats causing the the lack of change?

Dead Island and Just Cause 2 are the only ones I played with the 550 in. After trying to play on higher settings and the game running very badly, I ran benchmarks on Just Cause 2 and got the same frame rate on the same settings as my old card. After I read more about the new card and saw the 24 amps thing I just put the old card back in because I didn't want to mess anything up. I was also worried that my crappy CPU might be to blame, but my cousin (who knows more about comps than I do) said that card should have made a huge difference. My CPU is only 1.9 GHz dual core.

AMD Athlon 64 Dual Core 3600+ 1.91 GHz and 3 gigs of DDR2 (my system can only hold 4 gigs max).

The main reason i'm asking about this is because I was about to order a new PSU with the proper amps and better wattage. I just wanted to get a better opinion before I did anything.

The cpu is holding offMonsieurX

I guess it's all about your instantaneous power draw. Your lack of current may not seem a problem because you're not really stress testing your card. I would say that your card's maximum threshold is probably reduced due to under current. My advice is - leave well enough alone.

Your CPU is too slow.