The most cost effective upgrade you can make to that PC to improve it's gaming performance is to swap the GTX 460 for something better. 768MB of VRAM just isn't enough for recent games, assuming you intend to game at 1080p.
Your PSU is alright. Your CPU will do for now, unless you intend to play Crysis 3 maxed out. 4GB of RAM is probably sufficient for the moment if it's backed up by a decent graphics card, although I'd prefer to have 8GB installed .
I would consider swapping the 460 for at least an AMD r9 270, which start at about £110. That would give you about an 80% boost in gaming performance, and it has 2GB of VRAM (which is enough for any current game at 1080p).
Better value for money is the r9 280, which I've seen on sale recently for £135. It's about 15% faster than the r9 270, and has 3GB of VRAM. It uses more power, but your PSU should be alright. If you buy an AMD card from certain retailers you also get a choice of games from AMD's Never Settle promotion, making them even better value for money. The 'graphics card of the moment' is the nVidia GTX 970, but they start at about £240.
Hope that helps a bit.