Ok first of all, calm down, deep breaths
Ok lets start with XT. This is the most confusing, due to the brains of nVidia.
XT on ATI cards mean
SUPER DOOPER ULTRA good cards ON ACID, better than standard and pro.
XT (and LE) on nVidia cards means
REALLLLLLLLLLLLLY cut down, like the MX's of the GeForce series
GT and Ultra are pretty much confined to nVidia cards. Im pretty sure GT is like a "Pro" version and the Ultra ones are like the ATI XT cards - top of the line.
The amount of memory a card has can be a confusing topic to explain. Generally, confined to one type of card, the 256mb version should be faster, unless it uses slower memory modules. The game must have to use more than 128mb of the cards memory for it to be significantly faster.
Ok, so a 256Mb X800XT will be faster than a 256Mb GeForceFX 5200. A 128Mb 9800pro will be faster than a 128Mb 9700pro. A 128Mb 9700pro will, however, be faster than a 256Mb Radeon 9550.
And to reiterate the different versions of the cards, lets study the 9600 series:
Radeon 9600SE - basic version of the card - will cost you about £48 inc VAT
Radeon 9600 - The bog standard no thrills version
Radeon 9600 Pro - Tweaked version of the above, much faster than both
Radeon 9600 XT - a 9600 on acid! VERY fast, VERY expensive (compared to the SE) costs £110 inc VAT
And with your question about manufacturers?
If you are not overclocking, go for the cheapest, they are all essentially the same cards. If the advertised speeds are different, then take the fastest one, but i doubt there will be much in it. They should all perform nearly identically.
If you want to overclock, take the one with the beefyest cooling, OR take the cheapest and fit your own cooler.
Generally, before you buy a product, go to
www.hexus.net or any other site google finds and look for reviews of what you have in mind, so yoiu are aware of any issues it may have.
For the £200 you mentioned, if you can add a fiver to that, you can get yourself a Vanilla (plain) 6800 (nVidia), but theres usually a debate over what card would be best for the money...
If you are confused, this picture is a pretty good guide as to what goes where in the performance/benchmarking field:
Hope you find this of SOME use,
Regards
Chris