What games specifically? What settings aside from resolution do you have set for the games? Have you overclocked your video card by any chance? You can gain some performance by upping your memory clockrate on the Ti 4200.
As to BIOS settings, I'd say make sure you have at 32 if not 64MB set as your AGP aperature setting. Also, set your AGP Fast Writes to on and enable the AGP Master 1 Wait State (whatever it is called in your BIOS) options if your system can handle it and still be stable. Manually set your memory (mainboard DDR) settings to the fastest they will allow. When I was using the memory you are currently using I was able to set it at CAS2 with 2/6/3 on the timings. I don't have your motherboard so I don't know what your BIOS options are, but check your CPU specific settings. If you have something akin to Command Decode Rate set that to the fastest you can select. That's all I can think of right now for BIOS settings.
In Windows, make sure your system is defragged, that your pagefile is set to about 512MB min./max. (you have lots of memory, you might even get away with a smaller pagefile) so that you don't have that thing resizing on the fly which hurts performance often. Have you installed the Via 4-in-1 patch? If not, don't, just install the AGP patch, the latest one is 4.30 I believe. What version of the the Detonator drivers are you running? I use 30.82 personally as they are the most stable IMHO and offer good performance. In the NV control panel extension of the display.cpl, go to the DirectX tab under More Settings. Set the MipMap Level to Best Performance. I get a big increase in benchmark scores and game fps when I do this. I personally don't run games at this setting as my system is capable of playing all the games I own at over 70fps even at 1280x1024x32 with high details. But you may need this setting. How many programs startup with Windows on your system? Is your system tray full of crap? If you have lots of stuff starting up with windows, fix that. Allow only the essentials to load on startup so that you have max. memory available to the programs/games you want to run.
If I can think of anything else I'll post another reply. Good luck MiamiJones.