Upgrade path for my slightly dated machine

[himaro]

So, some years back i made my gaming PC that i'm still using to this day but it's just slowly becoming more and more under powered and i lack the proper funds to buy a full new system.
What i got it, it was as follows

Asus P5K Premium
Intel Q6600
2GB OCZ Reaper DDR2 RAM (can't remember the speed off the top of my head)
nvidia Gforce 8800 GTS 512
some huge heat sink from Tuniq

Over the years I've tinkered and updated it slightly with
another 2GB of OCZ Reaper memory
ATI 5870
Corsair H80
overclocked the CPU to 3.1GHz (standard Vcore)
Replaced the PSU when the last one decided it didn't need a capacitor any more.
i now have a semi modular 650W from Corsair.

and it's just not good enough.
FPS drops in almost all games, i'm now used to playing in the sub 20 FPS in many games
So, i have a cunning plan but i just wanted to run it past you guys first to see if you think it's really worth it.
I have an Gigabyte Z77X-UP4 TH that i won from eteknix.com a while back which i plan to use eventually

however, in the short turm, I'm thinking of buying a second XFX ATI 5870 to jam then in Xfire from Ebay (risk i know but meh, they got for about ?60 to ?90)
then, when i have some more cash but a I5 3570K and 8GB of RAM (probably Corsair) and transfer the cards and cooler.

with that, it should squeeze a year or two out of the rig before i commit to spending a few grand.

[NKrader]

My little brother still rocks a q6600 (stock clocks) for gaming and he does pretty okay, I'd personally buy a beefy GPU for that rig and it would solve most of your issues.

Also, possibly ram, look at your usage, I know with win7 x64 I used almost 4gb browsing the Web.