4gb 1333mhz vs 8gb 1600mhz

It depends on what applications/games you're running. You can check the requirements for most of the games (as an example, the recommended memory amount for Crysis 2 is 2GB and for NFS 2012 is 4GB). Virtualization, video or photo editing require more memory. Increasing the RAMs will not necessarily increase the performance of the PC.

All I do at the moment is web browsing and play games, but i might to do some gameplays for youtube and i'll use fraps and it records short two minute clips so i have to put them together with a video editor, but that's about it!

What motherboard, CPU, GPU and power supply do you use?

Motherboard - Gigabyte MA770T-UD3
RAM - Kingston 4GB DDR3 1333mhz
GPU - ATI Radeon HD 6670
PSU - 480w silent( don't know what make, but it cost me 15 quid so not the best!)

What about if i upgrade the gpu to a gtx 660?

AMD recommends a 400 Watt or greater power supply for a hd 6670. 480 might be on the edge. A better PSU is the right choice, IMO.

Better performance ( http://www.tomshardware.com/reviews/geforce-gtx-660-geforce-gtx-650-benchmark,3297-6.html ). Still a better PSU needed.

http://www.overclockers.co.uk/showproduct.php?prodid=CA-034-CS&groupid=701&catid=123&subcat=
Is that any good?

I would choose a 600 one, if no SLI intention for the future.

http://www.scan.co.uk/products/600w-corsair-builder-series-cx-cp-9020048-uk-80-plus-bronze-sli-crossfire-eps-12v-quiet-fan-atx-psu
It's a bit over my budget of £210 but it will have to do!
Thanks for all you're help!:)