There is a ton of information on TB about this - and a lot of different opinions on what is right. Here are my prejudices;
Most solid-state (not tube) amps are fine at 8 or 4 ohms, some go down to 2 ohms but you need to check. Cab manufacturers often rate their cabs at the maximum thermal dissipation - usually it is best to halve the power rating that is claimed. Typically, I suppose, most of us, given an 800W cab would not want to run more than a 300W head into it. This will be plenty loud enough for any small to medium gigs you are likely to do. 300W is a very common power rating for heads and you should easily find a wide variety of good stuff on the second-hand market for not a lot of money. I used a 150W head and a 240W 4x10 in a three-piece for years and never found it wasn't loud enough. When we did bigger venues there was always PA support. Even now I only have a 250W (old-school Trace) head running into a 1x15 and a 2x10 and still think is plenty loud enough but there are other here who think differently.
