now its the worst time to buy new hardware. arround september there are lot of new thing commin out ( i3, i5, 1156 socket, p55 chipset board, 890 amd chipset, 44nm GPUs...), these will drop prices on the older models. unfortunately, the i7 price will probably stay the same.
OC the Q6600 to 3Ghz (1066FSB / 333) - should be ok on stock voltage. Pushing it behond 3.4Ghz usually means a better cooler when gaming for a while as you have to push more voltage into the cpu ... consider it though.
Make sure you have 4Gb of RAM in it - even tho XP32 only uses 3.2Gig of RAM you might be using Vista or XP64.
Ensure the case has good airflow - otherwise you risk smoking up the 295 under load past a decent short gaming session.
A single GTX295 should be more than enough - in checking the value of putting a second 295 into essentially quad SLI I couldn't see much to gain on older games.
The money might be better spend on an i7 mobo and a 920 (overclocked) but you will also need 3 sticks or 2Gb DDR3 ... so the cost is something to grumble at there.
In summary, I would get a water kit like a cheap Corsair / Gigabyte (but not Thermaltake rubbish), and overclock the CPU to the max stable you can achieve under gaming, and top up the RAM to 4Gb.
If I were spending a few more dollars I'd get a second matching HDD and stripe (RAID0) the drives to double the I/O in that regard.
Might be the cheapest alternative to a mobo, ram, and cpu transplant.
The ol Q6600 can still keep up providing you can cool it.