Re: Low framerates...why???
Run glxinfo and look for direct rendering. If it says no you could have X libs with mis-matched versions, resulting in a performance hit. Do a backup and do some cleaning if this is the case.
There is NvAGP - the nvidia readme explains this including finding out which you are currently running. Whichever it is change it to the other and repeat your benchmarking (a reboot is a good idea when changing these drivers). The Sempron should do better with kernel NvAGP, especially if it is socket 754. Just that the kernel has to support this... so you may have to do a kernel compile.
Removing TLS files may speed things up just a tiny bit. If the above fails, search this forum for TLS.