Well its me again

OK its not the monitor after all, i went out and bought a new one today but the problem was still there ( new monitor is now back at shop ) so I'm thinking its more to do with my G/card ?? the games play well enough on this monitor and at lowish settings but when i play or more to the point TRY to play on the bigger monitors it goes nuts!! any ideas please i would be very grateful, also sorry if im cheeky, could you recommend a decent G/card ( £200 or so ). Thank you in advance.
AMD ATI Radeon HD 4870 1GB? would i have to change my ddr2 memory?