So just from nVidia sell the at least following models of their GeForce 5x00 range (this is all the ones Dabs currently stock):
|5200 Ultra||5600 Ultra||5700 Ultra||5900 Ultra||5950 Ultra|
|5600 XT||5700 LE||5900 XT|
(There's also several of the older GeForce4 range still available, and the newer GeForce 6800 range in plain, Ultra and "GT" varients).
Many models are available in both 128MB and 256MB varieties, with the very low end also being available in 64MB varaties. Generally, the higher the model number, the faster it is. "XT" for nVidia tends to mean slightly slower than the "plain" variety of the card, "Ultra" means slightly faster than the plain version. I'm not sure what "LE" means, it might be one from the card vendor rather than nVidia.
They're all fundamentally the same graphics architecture. The differences are in GPU clock speed, memory clock speed, memory bus width, and number of pipelines enabled on the card.
It's incredibly hard to work out what the best value-for-money card is, or even which card to buy given a budget of £x. This is especially true when you start trying to decide between a slower card with 256MB RAM and a faster one with 128MB. For example, a budget of £130 could get you a 5700 with 256MB, a 5700 Ultra (a fast 5700) with 128MB, or a 5900XT (a slow 5900) with 128MB.
ATI are just as bad.