That's a nice oem case; wish I could buy one for my board. For your upgrade, it's a crapshoot literally. Hard to say how high you can go for the cpu. The q6600 is liable to work fine, but without overclocking, which your board doesn't support, it will run at the same speed as your e6600. It's not worth a $100 gamble on a used cpu; though you could probably get your money back. I would sell your system intact when you save more money. Quad core systems in the states start at around $300 for the refurbished models with windows 7 and 3-4 gb of ddr3.
It says on that page you can upgrade to a e6600. Not a huge upgrade, but if you can find a CPU that fits your budget, it would be a nice little boost. That would be the processor in that list to get, it is the fastest, and the newest technology your system will accept.
Well it seems your pc can at max use a core 2 E6600 which they don't make anymore so your only option would probably be to use ebay to try and find one. There is also a Pentium dual core called the E6600 but I'm not sure if your pc can use it. I'd suggest standing pat until you can save up enough to buy a whole new pc, the upgrades you make really won't amount to much of a help if you ask me, what exactly do you use your pc for usually?
I'd run with what you got for a bit longer,, not worth spending money on this system,, once you are close, sell off this one to make the difference,,,
decent amd combo will cost about 400 bones and would run around this like mad,, another 150ish will have you in higher end intel territory...
You can access the BIOS. There's a setting near the top that allows you to set the multiplier. If you can set it, overclocking is supported. Otherwise, you may need to unlock it, though I have no idea how that can be done (and I've heard it's risky).