Build Advice £500 Futureproof (lulz) Upgrade

Howdy folks!

Looking to upgrade my gaming rig (CPU, Mobo, RAM, GPU) for around £500 (~100). I've sat for days in work doing my research but I'm at a loss. I'm looking for a setup that will give me 3 years of acceptable performance. My current system off the top of my head:

Thermaltake Tsunami case (£30 for unscrupulous shoppers at one point )
Athlon 4200+
ASUS M2N32-WS Pro AM2
4gb DDR2 GEIL 8500
Radeon 4850
OCZ SXS 600
Seagate Barracuda 7200 1tb

I've been looking at Overclockers' pre-OC bundles:
i3: http://www.overclockers.co.uk/showproduct.php?prodid=BU-035-OB
i5: http://www.overclockers.co.uk/showproduct.php?prodid=BU-023-OB
I know it's a cop-out but that warranty is alluring...

However as mentioned futureproofing is a concern, and these would leave me strapped for cash in the GPU department. I've read that Athlon solutions may be the way to go, but I missed the 6600 train and I'm still not over it! So I'm biting the bullet and asking for some expert advice (other than "OC it yourself you nab" - I am happy to do this myself).

I thank you for your time and expertise.

 

The warranty is good thing and they are all good parts i think you would need a 5850 to see a major improvement in gaming performance but day to day the i5 750 will really make a difference

 

This motherboard bundle seems to offer great value for money at only £250. That should then leave you enough to get a better heatsink & possibly a 5850 if you shop around. That would be a perfectly fine upgrade in my opinion.