Which graphics card would be best for this system?

I'm currently looking at gaming PCs on eBay and found a computer with the following specs. I've done a little bit of looking around but haven't quite got a real answer. The types of games I was hoping to play are: Sonic Generations Street Fighter X Tekken LEGO Harry Potter Years 1-4 and 5-7 Harry Potter and the Deathly Hallows (Parts 1 & 2) DC Universe Online 2x Intel Xeon 2.8hz 4 GB RAM 250GB SATA Hard Drive Windows XP Professional 32-bit It currently has this graphics card: XFX ATI HD5450 1GB And I plan on pirating Windows 7 Ultimate 64-bit when I get the computer. ^_^