Ok here's the specs from R* website:
Recommended specifications:
OS: Windows 8.1 64 Bit, Windows 8 64 Bit, Windows 7 64 Bit Service Pack 1
Processor: Intel Core i5 3470 @ 3.2GHZ (4 CPUs) / AMD X8 FX-8350 @ 4GHZ (8 CPUs)
Memory: 8GB
Video Card: NVIDIA GTX 660 2GB / AMD HD7870 2GB
Sound Card: 100% DirectX 10 compatible
HDD Space: 65GB
DVD Drive
To get that on Overclockers you'd be looking at something like £700 (£820 if you need to buy windows 7):
This as a Base, from their motherboard bundles section, provides the CPU needed, double the RAM (because windows needs some too) and a decent cooler, these are additional options:
Then this is the best bang for your buck on a GFX card at the moment IMO, this is slightly better than my current one (but I have 2
and is the same price as ones down the register a bit, but with twice the memory:
(To go from a GTX 770 to a GTX 780 which is not 'that' near the top of the tree these days costs another £120)
To power the above I'd imagine you'd need a Power supply too, something like this is a good option:
Construction Tech Support Free from either me or Ian, (you will also need windows 7 at least, not sure if you're using that already, this is another £117 for 64bit professional)
Edit: I just ran an approximation of a PC setup based on the above through a "Power supply Calculator" & it said 500w power supply minimum, I'd say 600w minimum and 700w recommended for some "Head room"