The GTX260 is comparable to the HD4870 and 5770, not the HD4850.
Because you plan on getting a new monitor, I'd recommend you get a card that is overkill for your current resolution, So that when you get your new monitor you wont have an trouble playing at its native resolution.
If you can afford a HD5850 go for it. But not just yet, might aswell wait a few weeks for nVidia's new cards to come out and hopefully start a price war.
Hate being on a budget, going over is gonna be tough aswell as i got mortgage payments and bills to pay. Thats why if poss i'd like to stay below £100. Was recommended the GTS250 1GB but with my board being an AMD Chipset will this throw up any compatability problems?