I have a GPU and wanna mine. It's an AMD 5850 card, can still get a few hundred Mhash out of it.
Don't care about profitability.
Should I go windows 10 or 7?
What AMD driver should I use? I heard that's key
Then, nicehash? Other pools?
If you want convenience and payouts in BTC, go with NiceHash. It's about as simple as you can get for set-up. However, keep in mind that with only a single 5850, it will take a
long time for you to generate payouts. A pool that can pay you more often will likely be a better bet.
Windows 10 vs. 7 is not a huge deal for a single-card setup, so choose what's most convenient for you.
As far as drivers go, you may have to look hard, since AMD stopped throwing in new optimizations for HD 5000 series cards quite a long time ago. Those haven't been common for mining since the days BTC/LTC dominated and were GPU-mineable.