Q6700 Upgrade/Downgrade vs. 9800 GT Upgrade

Hello! Currently, I'm running a lowly Q6700 at stock speed and cooling. Some what good at games, and works nicely for juggling applications, but that's just the four cores. From what I understand, I could downgrade to a Q6600 and overclock it, or buy an E8400/E8500 and leave it where it is.

Now, I could do that or upgrade my video card, which happens to be a 9800 GT, with the motherboard Intel G33. Yeah, I was surprised when I heard of it too - and it only came with one PCI-E slot - x16.

I'm in a bit of a bind right now, so I can only upgrade one side, the video card or CPU. I did want to use Nvidia's SLi feature, but that of course requires two PCI-E 2 slots, which I don't have. What I could do is pick up a motherboard that support PCI-E 2.0 for a slight boost of performance.

As you could of guessed, I'm not an enthusiast, but I would like to achieve as much in a price range of 250, but I can bump that up to 300. Should I change the CPU, Video card, or video card + motherboard for PCI-E 2.0 support?

Thanks in advance for informative replies.

Shorthand information:
Intel G33
Q6700
9800 GT
6 gigs of RAM
Air cooling (I'm cheap tot he point of just setting up a household fan to slow directly on the exposed components of the computer.)

Cooler Master 500 watt

http://www.newegg.com/Product/Prod [...] 6817171031

Yeah shubham is right that 500w coolermaster isn't going to be enough to power a GTX 275. Since the 9800GT is still a viable card for todays games maybe just upgrade the CPU cooler and get a 750-850W powersupply and wait another 6 months for the DX11 cards to come out then get a Radeon 5870/GTX 300 card?

EDIT: Sorry, double post.