Might want to list the parts. Just press enter, or use the bullet points. Will be easier to read.
A pentium and a 7770 should play quite a number of games. It really depends on the experience that you want, the quality that you want, the specific games that you wish to play. The £300 setup would certainly struggle to play some games.
If you're looking for something that's upgradeable, you need to specify that. Give as many details as possible.
Refer to the sticky on the build-a-pc forum. It outlines many questions which would really help us both.