Many ppl use win7 for cf play ... its also the most tested windows version for cf 2.0. Putting aside used win version, majority of issues come from uac and antivirus.
Greetings, go for both - start SP (campaign) as also MP. In MP you can get tips and advices much faster and with this you can pass SP much smoother (check CF wiki for starting guide). For graphics imo the best choice is sweetfx.