Graphics card reccomendation(s)

hey i am new here so bear with me,
i am building a pc atm and have bought my graphics card (XFX ATI Radeon HD4870 512MB) and my motherboard (MSI 790FX-GD70) , i am looking to quad crossfire, but i only want a second one to start off with , i will add more as graphics get more intense. so any recomendations welcome

P.S i have a budget of about £100 - £150

oh and btw plz let me know if 2gb is enough ram for games on xp?
xD

realistically, you'd want to stick with the same line -- so you should get another 4870. you don't have to, but you should stick with it.

ok,
what are the dissadvantages and advantages of putting different cards in crossfire:
lets say 4870 & 4850?

you'd run at the spec of the lower rated card

http://ati.amd.com/technology/crossfire/faq.html

oh ok not really worth doing then?

no all cards should be in the same line. I also would recommend identical cards. e.g
if have a XFX HD4870 u should also get another XFX HD4870 and not Sapphire HD4870, just to advoid any problems.