Which PSU shall I get?
XFX 550w - £51
Corsair 600w - £54
Which shall I get will the extra 50w help me out in the future?
EDIT: I am adding a HD 7950/7970 to my system. This means the xfx will only just have enough, so should I get the corsair?
The XFX easily. It is a step up in quality over the budget level CX.
The XFX has more than "just enough" power. Your CPU & GPU draw their
power from the +12v rail(s), a system with a 7950 needs 25amps,
7970 needs 30amps, add 6a to each if you are going to stress test.
The xfx has 44amps on +12v rail(cx600 has 46), so you have plenty of headroom
for either GPU. So, go for the better quality.