Upgrading graphics card on slimline PC

Hi there,

Posted this in a thread, but think I'd get a better response here. I've got a 2 year old Media Center PC that is okay for watching standard DVDs and low spec games, but starts to jerk with HD content and games set to anything but minimum settings.

When I got it I was told I may find the video card to be lacking...but didn't upgrade because I didn't think I'd need superior capability.

The spec:

1. Main use would be as a media center...find HD playback to be a bit jerky right now. Would play medieval total war 2 if it wasn't so slow.
2. About £50 ($100)
3. PSU - 180W 10A from "FSP group". But it's a micro ATX form factor and I've found it near impossible to find a replacement.
4. MB - 915P-M3
CPU - P4 3.2Ghz 640
Memory - 2gb DDR
Graphics - PCI-express, got a Radeon x550 in right now
5. From the UK

Whats the best graphics card I can get for my system without bottlenecking or having to upgrade my PSU?

Thanks,

~N

Anyone?
....