Windows is needlessly big, but if you get past that a 128gb drive can take windows, a bunch of programs and 2-3 big games, you'll find loading speeds on most games won't matter and if you try and put a bunch on there you'll run out of room eventually.
I'd go with a 128gb for now and maybe, when hdd prices return to normal look into something like a hybrid drive, which are not at all bad in terms of performance. At the moment due to costs something liek a 128gb ssd for windows and a couple games, and a 750-1tb drive with a small ssd cache looks to be the best way to go.
As for which 128gb, or 256gb to go for..... honestly you won't see a huge difference between any of them, and they all have their issues. The M4 to date has been significantly more reliable, and Crucial have for a LONG time been very easy and quick to RMA with in the UK which is fairly useful. M4/Vertex do beat the Agility in certain situations but you'll likely never "feel" that in real use. Considering the prices, with the number of the Agility style drives from ocz/corsair you see on special offer for around the £120 mark its probably a saving worth making, if you spot a M4 on special between £120-130 which has happened, I'd go with that.... obviously that would be 128gb models
Pretty much 128gb is "liveable" for me 64gb isn't, and 256gb would be great but ultimately very expensive. it's likely in 3-4 months the seagate hybrid drives will be down in price dramatically and I would think everyone else will be coming out with similar drives, or you can buy a drive specially for cache but its a bit overkill and awkward, the all in one solution, and size, make the Seagate a nice option..... but I'm not sure what the price of the 750gb would be post HDD's returning to a normal price.
