This is your board so you have an AGP slot click here so you need an AGP graphics card.
Both cards you've listed will fit and theres a massive difference, perhaps you've worked this out yourself bearing in mind the price difference, the X1950 pro is one of the best AGP cards you can get and the 6200 card is very basic.
You will need to check power supply rating as the x1950 will need at least a 450 watt power supply and I think they all need their own power connector as well so make sure you have one spare.
One thing I'm a bit unclear about. You mention £50 as a budget. Where did you see a X1950 Pro for £50 as its a move than £100?
The 1950 pro is a much better card and costs about £108 new from Novatech, depending on how old it is the £60 your friends wants could be a good deal. You can get it cheaper elsewhere though maybe £80.
Anyway. The card JoeR linked to the 6800 is quite a bit better than the 6200 you mentioned without going overboard.
All new AGP cards are 1.5 volts as far as I know you cannot get the 3.3 volt ones anymore.
In short, any of the cards mentioned will fit and of the 3 mentioned the 6200 is the poorest the 6800 is quite a bit better and the X1950 pro is a lot better.