Overclocking is not dangerous these days. Just take it up in increments of 5-10MHz at a time, each time testing for stability. When you have found the max it will run, without having artifacts shown....lower the overclock a little to be on the safe side.
The amount of memory on a graphic card, mostly depends on what resolution you are playing at. If you play at high resolutions, such as 1600x1200 and up, then 512mb will come in handy. Resolutions like 1280x1024, a 256mb graphic card is ok.
Yes you can....but the 7900GTX is too expensive, when you can get a cheaper card, that will perform close to or better. If you're spending that much on a video card...I would go for what Sora mentioned....the X1950XT.