My initial thought was to maybe add in a SSD to the system. I would give the biggest overall "perceived" effect on system performance, but would be a 100% spend as I have no previous SSD to offset cost. So estimated cost would be about £80.
Or I could do a core platform upgrade. Get in an Ivy Bridge motherboard with a new third gen I5, keeping the RAM. Estimated cost with offset from selling old things: ~£100.
Lastly I could sell on my 5970 and look at getting either a 7970 or a GTX 680. Looking at the UK prices at the moment, the 7970 looks like an incredibly compelling offering with it's price/performance attributes. £320 for the cheapest one on Scan. The 680 is looking at about £400 retail, which is a sizeable increase. I've looked at some benches for the 680 and they really do rule the roost, but enough to justify £80? I'm not big into OC'ing my graphics cards, for the record. The most I'll do is set a more aggressive fan profile on Afterburner. If I sell on the 5970 for about £200, which is what they seem to be fetching on eBay etc, the cost will be looking like £120 to £200.
The only reason I'm approaching the graphics card issue with some trepidation is the fact that my 5970 is still breezing through current games fairly easily. Just get the latest drivers with the app profiles in there, and you're set. Sure, it's not full settings, but most of these games still look damn awesome on medium settings.
Is there something in particular that is not performing as well as you'd like? That would be where to aim the wallet-cannon.
Depending on how often you're in and out of applications, a 120GB SSD is probably worthwhile. I'd choose a non-Sandforce drive like a Samsung 830.
If you're looking to make improvements not specifically related to objective performance, consider other things like your keyboard and mouse, chair, desk lamp, etc.
Thank you for that link. Looks like I may as well wait for Haswell as my I5 is still tearing through games (4GHz ain't too bad!). Looks like I'll be looking into a SSD then, The Samsung 830's are looking nice.