No he was talking about crossfired HD5770s. One wont come close to an HD5850 even when OCed. In any case if you get an HD5770 don't even think about crossfire until you get a higher resolution monitor. One HD5770 will already be a large amount of overkill for 1440x900.
The HD5670 should really be quite good at that resolution as is but you will definitely see an improvement from an HD5770, especially in the more intensive games(Crysis, Metro 2033, Stalker, BF:BC2, ect.) Whether it is worth the money is really a judgment call.
The ATI 5770 is a great card. Essentially equal to the GTX 260 Core 216. It'll play most games fairly well up to 1920x1080 as Shadow suggested (especially in Crossfire). Since you're using a 1440x900 resolution, a single 5770 should own anything you throw at it.
Thanks for the reply guys, so you would say that getting a 5770 is better than spending just £70 on getting a second 5670?
Crossfring 5670 will give you 5770 performance, it has literally half the sp, tu, and rop's (have to check) , there are a couple 5670 crossfire reviews, in some games its a little faster than a single 5770 in some a little slower. Those are all games that support crossfire, which most do. But there are always some that don't, and thats a drawback of all dual gpu rigs.
Trying to get a few bucks for your 5670 and getting a single card would probably be better and leave you more options for later. http://www.guru3d.com/article/radeon-hd-5670-review-tes...