Dell Dimension E520 graphics card upgrade help

Hi there,

been around these forums, and used them before but would hopefully like some help again!!

I have upgraded my DEll E520 quite a bit, but im looking for a new graphics card to give me a good grphics and less laggy experience on games.

My current specs are:

600W PSU, Intel Q6600 2.4GHz Processor, 1Tb Hard drive, 4gb ram and curently a ATI Radeon HD4650 512mb graphics card.  Running Windows 7 Home Premium 64bit.

Obvious worries are the single pci express slot, and the relatively small space for a graphics card.  But is there any decent upgrades i could obtain for this card for say around and under £100 that would give me a noticeable difference,

Cheers for any help