I built my current system for £850 two years ago (with small additions since). Currently it is:
i7 950 @ 3.2GHz
1.28GB GTX 570
8GB DDR3 1333MHz RAM
MSI X58A Motherboard
128GB SSD3 via SATA2 (as the motherboard has that dreadful Marvell chipset for SATA3) (backup 250GB regular HDD)
Coolermaster 650W PSU
Used for games, performance is so-so: less taxing games can be run at any detail but demanding games such as Crysis 3 hover around 40-50fps on medium settings. (I'm super fussy about low-to-average frame rates, struggle to enjoy a game when it's almost-imperceptibly less responsive)
I believe the CPU is a bottleneck. Even with a beastly cooler that makes a huge amount of noise when working hard (Arctic Cooler Freezing 13), I've found 3.2GHz to be as far as I can go (and even then, heavy stress can see the CPU push 80deg. (CPU was mounted with a fresh, thin layer of grease))
Although I don't follow hardware tech unless I'm upgrading, I believe the GTX 570 is still an okay card? The only issue with the card is that the manufacturer (Inno3D) under-volt it, which means I have to have MSI Afterburner running the card at an extra 25mV, which is pushing the limits of my understanding about maintaining a system. I've checked tomshardware's GFX card hierarchy chart, and the GTX 570 is (off the top of my head) around the 5th tier, and the article only recommends upgrading to at least 3 tiers higher- pointing to out-of-my-price-range cards.
A quick 10 minute scout on ebuyer has me looking at:
- i5 3570k @ 3.4GHz
- Be Quiet Shadowrock Top Flow cooler
- MSI Z77A-GD65 motherboard
So would marring those upgrades to the GTX 570 + 8GB DDR3 RAM (as well as allowing the O/S-mounted SSD to operate at SATA3 speeds) be a worthwhile investment? I'd probably look at pushing the CPU to 4GHz.
The i5-3570k is the most performance any gamer could need, but I think your i7 is nothing to sneeze at. What games are you dissatisfied with performance-wise - there are games that need the additional CPU muscle (RTS/MM680 ORPG) and there are games that need the extra GPU muscle (FPS/Action).
In your shoes, I would try to find a buyer for your 570 and let that determine your GPU budget. A 670/680 is going to be more than enough power to get you through all current gen games on Max settings including AA/motion blur.
Getting your SSD connected to a SATAIII port will be a difference maker in terms of day to day performance (typically doubles read/write access times) but it won't be a game changer for gaming performance so it's not an immediate need. I think a processor and mobo swap can be on the back burner.