Radeon X600 upgrade path

My Hewlett Packard m1090n (3.6GHz, upgraded to 3GB ram) has a defective
Radeon x600 video card. Since I have to replace the card anyhow, I would
like to utilize this opportunity to upgrade to a higher end video card. I
should probably stick with the same brand. So, what would be a suitable ATI
card? It should be PCI Express, and the computer has a 300w power supply. I
could replace the power supply, but would prefer to avoid doing so. Lastly,
I don't have an empty slot below the video card, unless I move the soundcard
to a different slot. I run mostly business apps, do a lot of email and web
surfing, and do some video editing with Adobe Premiere Pro. Any suggestions
would be appreciated.