Ok , So I found a GTS 450 at a sweet price of 219 the price of a decent GTS 250.
What would you think of the DDR5 GTS 450 ?

Also. If I get that and a new PSU. Would I have to upgrade my DDR2 Mobo aswell ?
and would the CPU need upgrading aswell ? 

If you're happy turning the settings down to medium and not using AA and AF you could go with a Nvidia GTS 450 or Radeon 5770.

When you are actually playing a game you usually don't want the frame rate to ever drop below 30fps or it will be noticeably jerky to the human eye.  Conversely consistently high fps over 60fps means you can probably turn the settings up for better quality/detail.

Here's a good review that looks at real world gaming performance (I really like they way these guys benchmark) and not just average frame rate for the 450.

As you can see it would be a good choice for 1680x1050 resolution screens but in my opinion you need to look at the next model up with a 1920x1080 screen if you want to play at high settings in new games.