Well the Ti4800SE is a GF4 Ti4200 that is just runs at 8x AGP, as opposed to 4x. This makes very very little difference, perhaps 1 or 2% in terms of frame rate. The Ti4800 standard is a Ti4600 which runs at 8x AGP, again it's only a 1 or 2% gain on the Ti4600 model.
Ti4200 64MB cards have faster memory than the Ti4200 128MB models (500Mhz DDR versus 444Mhz DDR). Because most games don't require 128MB of video memory, it's better to get the 64MB version which has less but faster RAM.
The Ti4600 was and still is a very good card. Reliable, compatible and even today is still very quick if you're not using full-scene anti-aliasing and anisotropic filtering.
You should be able to pick up a GF4 Ti card for around £50 second hand.
The GF FX range is rather interesting. The FX5200 and FX5600 are both very poor when compared to similar cards in the same price range. They are not particularly quick at all, and despite supporting DX9 features, both cards will be too slow to take advantage of any DX9 effects used in games.
The FX5800 was a short lived card, made in very limited numbers. It performs similarly to a Radeon 9700 but was quite a bit noisier. It's not a bad card, but you would almost certainly have difficulty tracking one down, and in any case the Radeon 9700 is a better bet.
The FX5700 is also not a bad card, it is mid-range and should cost around £130.
The FX5900 and the FX5900LX are both very good cards, PC world were selling the LX model for £90 on boxing day last year! Excellent value.
In conclusion, for best bang for buck at the lower end of the market look out for a second hand GF4 Ti.
