Dell Inspiron 530 (bought it few years ago)

3GB RAM
500GB HDD
32 bit vista
Intel core 2 Quad CPU Q6600 @2.4GHZ

But graphics card is ATI Radeon HD 2400 Pro

My question is, what is the BEST graphics card I can upgrade to?, but like ONLY changing the graphics card, and nothing else. Trust me im no expert on PCs

Im from UK, so if anyone can link me to some budget graphics card for a 300W power supply, that would be great. Im not planning on replacing my power supply, I just need a slight graphics boost.