firstly
coldtony: Devil's advocate. Build your system with an AMD APU like the A8 6600k (quad core, overclockable), which has integrated graphics (equivalent to HD6670) probably better than what you already have. Run that for a while and see if you even need a discrete GPU. By the time you decide whether you do or don't for sure graphics cards will be quicker and draw less power anyway so it's win win.
edited because of some of the points raised below. The A8 6600k runs most current games at medium settings/1080p and is about the equivalent of an i5 3570k for less money.
I would really hope that anybody who pays attention to that takes a really good long look at exactly what those chips are capable of, and all the limitations that go along with them. many would tell you to get a pentium and a low-end gpu like the AMD 7790 or nvidia 750 if your focus was gaming, sooner than that APU. in addition, the unlocked pentium is coming out soon, or is out already, so that's something to consider as well, when pricing a pentium and low-end gpu vs an APU.
on the i3. the haswell i3, 4330 and also the i34130 I think to a degree, show very strong results in all current games but a few particularly demanding games that are designed to be demanding. those two chips practically match the i5 4430 in the lions share of benchmarks. the haswell versions, remember. but it is true what people say about needing 4 cores for the future. with the consoles built on low-clock, high-core processors, the software will continue to make better use of multithreading capability. 4 fast cores in place of 8 slow ones could be necessary, and the i3 doesn't have that. it could be the i3 starts to struggle in the future, it could be that it holds up like it is doing now as Intel ever increases their IPC. I would recommend the i5, too, but only if you can afford it. if your focus is just games, then you are better spending on a better gpu. if you don't like the higher power draw of the better cards, period, then maybe spring for the i5 even if it's not in your budget.
another important thing is that 20nm hits next year, and there could be a measurable jump in gpu performance. I can guarantee you nvidia is just itching to force another performance situation. I don't think it will be a total crapshoot, because DX12 will support current-generation cards, so they know it's not gonna go like it usually does. but it could be that 20nm makes a difference, and it could be current cards get huge price cuts when 20nm hits.
building a PC is unnecessarily bothersome. the only way to be sure you will be happy is to know what you're doing. the less research you do the more you need to pay attention to build guides and the like. if you don't know what you're doing, the PC market will burn you. that said, that 750ti is a good buy. it's pretty good for the money when you look at the performance, the awesome, awesome power draw, and noise and heat output. that's if you can get one that isn't marked up to high hell by the retailers. the MSRP was supposed $150.
On W3, I think the 750ti won't do it any higher than medium. I can't say whether that's with 1xMSAA or not but it might even be not.
nevermind, I made a reminder of microcenter but I see you live in the uk