Just so that you understand where I'm coming from, I'll give you my current layout.
I have a pair of 2x10 cabs - one is an Acme Low B2, the other is an SWR Goliath Jr III. Both are rated at 350 watts RMS and 8 ohms and it also turns out that they play together a-okay. And I only need both of them here and there - not all the time. When I do need both, they give me big sound - I'll set them up in a vertical stack.
My heads are a GK MB 500 and a Genz Benz Shuttle 6.0. Either head is plenty for any indoor gigs I play with a rock and dance cover band (using both cabs together), but if I play outdoors, my preference is the GK. It's just a little more eager to get really loud compared with the Shuttle. I initially considered getting the MB 800, but after a few years of gigging the 500, I now know that the 800 would have been way too much for me.
So with that in mind, I'd say that you should be fine with the 500 as long as you have the right cab or cabs to deliver its sound at the levels you need. Only 200 extra watts may not make much of a difference for you in terms of pushing more sound if you don't have enough decent speaker to shake enough air for you. I can play smaller indoor shows using only one cab with either head no problemo, but only one of my cabs isn't enough when I'm outdoors (without PA support) using either head.
Now that I'm getting better at running some of my sound through our improved PA, my amp doesn't need to blow as much of my sound off the stage and into the venue. It primarily serves as more of a monitor and our stage volume stays lower this way. With some of my sound going to the board, I can also put it into my drummer's monitor mix to help him hear me at more moderate volume. If a PA will be helping you out, you should only need enough cab to let you monitor yourself on stage or for practice at home or with others when you jama-lama.
