+1 for thingemajib's suggested Q6600 + 5770 upgrade
Thanks for the replies guys - three further questions if I may...
1. Is there anything to choose between the various manufacturers for the 5770?
Yes, some do use different quality components, different cooler types, BIOSs and offer different terms of warranty. I strongly suggest going for the
ASUS CuCore 5770 - as it has a non-reference cooler (which is a bit better than the stock one), the BIOS allows you to "voltage tweak" the card for really good overclocking and it comes with a 3 year warranty (which is 1 year more than the rest).
2. Are you able to recommend somewhere to get a Q6600 second hand (not eligable for the Members Market here

)
Unfortunately not, we can't link or suggest competitors - we can only say "have a google". However, your forums account has 102 post (and more than enough time), you may want to consider getting involved in some discussions (GD and speakers corner are always great) and ask some relevant questions - who knows in a weeks time you may have the 250 posts you need to snap up one of the Q6600 that regularly appear there.
3. Will these upgrades give a significant boost? (the current system has been fine until recently - even ran Crysis on low/medium quite happily - but the poor old graphics card falls over after a minute or two of Borderlands)
Massive boost. Not only will you be able to play games with higher detail levels, but your framerates will be much higher, giving you smoother gameplay.
You may also want to consider selling your E6300 CPU - for the price of selling that you may be able to buy a nice CPU cooler (like
this) and give your Q6600 a nice overclock.
Finally, make sure you update your BIOS to the most recent version before you install the Q6600 - as earlier BIOSs won't support the Q6600.