Best Gfx card for athlon 64 xp 4000+

Hi Guys

Hope you can help with this.

I'm looking to get a new pci-e Gfx card for my system:
Asus A8N-e mobo
Athlon 64 XP 4000+ San Diego CPU
4GB RAM
550W PSU

I'd rather go the AMD route, around the £100 mark but also looking to upgrade PSU as I think existing one is a cheap build.

Finding it really confusing as to what my system will support and not cause bottlenecks etc...

Thanks in advance.

Rich