what type of games do you play? the hd6450 and hd7850 are in different leagues the hd6450 is the slowest and weakest dx11 gpu in the market whilst the hd7850 is actually a decent high mid range performer. Now the question is- do you need it? It really depends on the games you play and the settings you use. In the future, the value/performance ratio of the gpu would get better for sure so you might wanna wait until you actually need a new gpu and your current gpu fails to run the game you want to run. But then again the hd6450 is the weakest card in the market right now and even the intel integrated hd3000 and hd4000 graphics perform better than it. So I assume your running all the older games at low settings. For that, you dont need a new gpu.
yes the hd7850 performs about 2x the hd6750. However, the hd6750 is a decent low-mid range card so like I said before, if you can run all your games properly with it- then you should keep it. But if you plan on running dx11 titles like battlefield3, batman arkham ciity, crysis3- then upgrading to a hd7850 would be the way to go.
Its a matter of personal opinion. Personally, I like pre overclocked gpus even though they cost a bit more. Those card usually have better quality, runs cooler and more headroom for overclocking- the premium you pay for them is definitely worth it. For h7850, I'd recommend a sapphire OC model or asus direct cu II. If you can find them within £10-15 of your xfx card, then you should get them instead.
AH, thanks shamsmu,
one game I'd like to get is War of the Roses, but I checked on CanYouRunIt, and my GPU was letting me down, I'd also like to upgrade for Far Cry 3, so I'm gonna save up, I've seen a Sapphire 7870 for about £30 more than the XFX.