Depends. A Pentium D is a sufficiently slow CPU to bottleneck almost any graphics card you buy. It depends what you need the PC to do. No need for a $300 high-end card if you'll only be playing basic games, but equally you'll still find situations where your old graphics card may be a limiting factor, but not the CPU. The most it's probably worth upgrading to for that system is perhaps an HD6670 or so. Anything beyond is going to be a bit of a waste without upgrading the CPU.