You have ~750fps with glxgears - wow! I only got ~650fps but then I'm
running a 1600x1200 rather than 1280x1024 display. I think the issue
is memory bandwidth - glxgears is a test of memory bandwidth more than
anything else - there is no fancy shading so all of that DX9 hardware
counts for naught.
FX 5200 cards come in two flavors 64bit and 128bit memory bus and I'm
sure I have the 64bit version (notice how card manufacturers are keen
to list tons of mostly meaningless trademarked buzzwords on the box
but are less keen to list important things like GPU clock and memory
bus width and clock...) - to be fair Gainward did have the word "lite"
on the box.
With so little memory bandwidth and a 1600x1200 display sucking a
large chunk of it, 650fps is not so surprising. I tried an FX 5600
card and got ~2350fps so I'll be returning the FX 5200.