Enough space for your games? I think you are confusing RAM with on board video memory, two different things. You don't need a $300 card to play sims 2. Sims 2 will play on a vid card with 32MB on board memory.
What kinda vid card do you have? cpu? RAM?
OK, sorry for underestimating your knowledge... you can keep your case, power supply, hard drive, optical drives etc. and just get a new mobo, cpu, vid card and ram, that would pretty much be a new computer. You could install it yourself, it's just a couple screws, some thermal paste and new drivers. If you have to pay someone to do it it might be better to just get a new rig.
wait.. I'm assuming you need a mobo upgrade in order to upgrade your vid card... if you mean it will cost $300 just to buy and replace the vid card in your system, that's ridiculous. You can change out a simple vid card, that's cake. It's only 1 screw and install new video adapter drivers.
What kind of motherboard and video card is it? If you're running on integrated video, there is hope to upgrade. If you have an actual vid card that is less than 32MB, it's probably a 1x AGP mobo and your upgrade options are limited