Up-grading Dell Inspiron 530s Graphics card

Hi all,

I (regretfully) bought a Dell Insprion 530s last year. It has a slim line PC tower so all the componants inside it have to be slim or low-profile. I currently have an ATI Radeon HD 2400 Pro with 256Mb of memory. It also has a Intel Core 2 Duo at 3Ghz and about 3Gb of DDR2 SDRAM.

I really want to upgrade my graphics card to, as good as I can get, for maximum improvement. Now i know not all graphics cards will fit inside of my tower and I really don't want to get a new tower so the graphics card has to be a low profile one. And i know i won't gain the full benefits from buying a rediculously powerfull one if i dont upgrade everything else.
So all i want really is a good upgrade to my current card. Only thing is I don't really know what to look for or where to look for it. Money isn't really an obstacle either as low profile cards aren't too expensive.

Cheers for any suggestions!