1. Mostly around 100 and up. The real catch is that you'll probably be wanting to run double cards. Some high end machines will even have a separate graphics card set aside to do some rather specific tasks.
2. I'm really big on keyboard/mouse, but as for as controllers go I've used a ps2 with a usb converter. I don't do it often though, so I can't really comment on any problems.
3. Like everyone said you can use paypal, however, I would much rather just use steam directly. Paypal has been kinda "ugh" lately. It's not like steam is not trustworthy.
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Chrono1219