The GT 520 would be your best bet. It isn't high-performance by any stretch by today's standards, but relative to your hardware, it might fit the bill for "high performance."
http://www.newegg.com/Product/Product.aspx?Item=N82E16814500233
Just get a new PC.
You don't have a quad-core if you're on DDR1 RAM. The first real quadcore was the AMD Phenom, which used a DDR2 memory controller and wasn't produced until 2007. You have a dual-core at best, but I'm guessing you actually have a Pentium 4, given the clockspeed.
Just saw the update. Anyway, recommendation of PC replacement still stands. Any decent GPU by today's standard would be limited by bandwidth, bus, and CPU. But if you're hellbent on keeping your current PC, the GT520 is the fastest PCI-e x1 card I could find.