Decide on what size you want first. Then decide how yellow you're willing to go. Most people will be happy around H or maybe I, anything more is noticeably yellow. Although face-up in a ring you may not notice at all, particularly with a brilliant cut diamond.
The proportions determine how well it sparkles and the fire - probably not something you'll really notice and most are cut with good proportions. Watch they're not way out though.
As for clarity, as has been said - VS1/2 is very good value for money. In fact, you'll probably not see SI1/2 at all either and that would make the diamond considerably cheaper.
As a final note of warning, just 2 days ago I had to return a diamond worth 5 figures after getting the stone recertificated and discovering that it was a much lower spec than the original certificate.
Only buy stones with a recognised certificate e.g. GIA, IGI, HRD, EGL etc.