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From http://www.notebookcheck.net/Computer-Games-on-Laptop-Graphic-Cards.13849.0.html, it definitely is an improvement, though you could do better, much better.

Aren't those mobility chips on the benchmarks? Seemed like he was referring to desktop versions.

Doesn't seem like a big enough jump though to a 550ti - I'd prefer just buying a second 4850 if he already has capabilities to add another one.

They do desktop ones as well, just click on the desktop graphics bit on the circles and than look up the graphics along with the games fps you want to look at.
He should save up for something better.

XFX ATI Raedon 4850 512mb or Evga 1GB 550ti ?

A 6850 can be had for a little over £80 or £70 secondhand. That would be much better than a 550ti.

Having recently put a build together using new components apart from the graphics card (reused HD5770) I feel it might be an idea to upgrade the graphics card. Is there anything better within a £100-150 price range?

in order of price:

6850, 6870, 560ti

power:

560ti>6870>6850
.

Would you recommend any of those on a 500watts PSU? It's an Antec EA500 bought in 2007

6850 is under £100 and minimum AMD suggested i 500 watts.

6870 and 560ti should also work with 500 watts
hope the power supply is certified for efficiency.

It says something about being certified on the PSU itself

Do you have any experience of a second hand GPU? Kinda wary as it is second hand

Yeah. I bought a used 460, it was fine. I also sold a 6850 and it was in perfect condition. I was trying to buy a used 7850 but they're too new to have many used ones out there that are cheap enough to forgo a new one. So I just bought a new one.

Better off buying one still with warranty left. They're very unlikely to be dodgy as the original owner could just RMA it and get a brand new one.

Is the Cooler Master Cosmos S a good tower case, contemplating getting that or the Corsair 600T Mid-Tower Case White Edition.

Is the Cooler Master Cosmos S a good tower case, contemplating getting that or the Corsair 600T Mid-Tower Case White Edition.

there both pretty good, the insides of the Cosmos seems much nicer

Yeah, liking the Cosmos coz it looks like a tank, but the Corsair looks really sleek, confused! :P

What about the Cooler Master HAF series?

Where would you buy them from? eBay?

I know what you mean, it depends completely.
Cosmo is more about slience, portability, everythings in an organised manner.
While the Corsair is very strongly about good airflow.