There are ways to unlock the Radeon X800 Pro into a 16 pipeline configuration. However, I believe your card has to be of the VIVO model for that to work.
http://www.google.com/search?hl=en&q=unlocking+x800+pro
Your problem about the fan cutting out and the game lagging is quite an odd situation. A fan slowing down will not directly cause a game to run worse. It may be possible that the video card is thermally throttling, however I have never heard of thermal throttling taking place on anything other than a CPU.
I suggest you get all possible Windows Updates, all of the latest drivers, and make sure your pc has no viruses or any other form of malware.
I do not suggest upgrading from an X800 Pro to even an X850XTPE(The fastest AGP card). This is because for the price you spend, you would get at best, 25-35% better performance. That would not be worth it in my opinion. I suggest you wait until you can afford a PCI Express based platform before you upgrade to a new video card.
Are you using an AMD or Intel based machine? If you are using an AMD based machine, I suggest this mainboard as an upgrade:
http://www.newegg.com/Product/Product.asp?Item=N82E16813157081
This mainboard is socket 939. It supports both AGP and PCIe based video cards. This way, you could upgrade to that mainboard and still keep your X800 Pro if you do not have the funds to upgrade to a PCIe video card at the same time. The board supports all socket 939 processors, including dual core. The board also has an add on slot for the upcomming AM2 socket based processors, so you have some impressive product life on that platform.