You screwed the pooch on this one. The LGA775 socket is a dead socket. You've locked yourself into a lot of old tech without much of an upgrade path. It *might* be alright in some games, but you're going to have some trouble with trying to play the newest ones at the best settings. Photoshop will be performing horrendously (at least compared to what you could have gotten).
Luckily, even with the 600 pounds you have left (plus whatever you can sell that stuff for), you can still get a good computer. Try pricing this out over there (the site the links and prices are from won't ship internationally):
DDR2 RAM will not work with either of the mobos that have been suggested to you.
The GTS 250 is a bit weak for gaming, depending on the resolution, so I'd check it against some benchmarks of games that I would be interested in playing. However if gaming isn't a high priority it will probably be fine.