It's a not bad motherboard. The biggest problem is that the G41 chipset is an economy chipset with a limited FSB frequency. You will not be able to push the FSB past about 350 - 360 MHz limiting your potential overclock to no more than 3.2 - 3.3 MHz.
I have a couple of G41M-ES2L boards that I really like, but I am unfamiliar with the tradeoffs that a combo board using either DDR2 or DDR3 RAM has.
Most Q6600's will run at 3.0 GHz with little or no voltage increase with the stock cooler.
I suggest that you go ahead and install the 5870 in your present system to see how well it works. If it is "adequate", save your money for an AMD BE or i5-K system.