I agree with the 400 watt deal. If your card is just about running on a 250watt (i'm surprised), 500watt would be overkill
GeForce 6200 GT - This card would be fine for playing games on by the way at around low/medium settings. Now, if you wanted to run at Medium/high settings in games, try looking for the 6600 GT which costs around £170 in english pounds, so about $210 I would think.
That card would give you some great performance boosts!
I hope this helps