Any 1 of your cabs will give an 8ohm load. Any 2 will give a 4ohm load.
The difference is that when the cabs have a higher impedance the amp will be a little bit quieter for any given settings - so More cabs = lower impedence = a bit louder.
As your cabs are 200W rating or 400W with both you couldn't crank many modern amps all the way up, so you'd have to be sensible on the volume control of a higher powered amp. Not a big deal though. My cabs have a total handling of 700W and I use a 900W head.
Your budget is plenty for a good choice.
If I was you, I'd look at this Streamliner 600 : [url="http://basschat.co.uk/topic/214165-fs-genz-benz-streamliner-600/"]http://basschat.co.u...treamliner-600/[/url]
Vid review of the 900W version:
http://www.youtube.com/watch?v=d-27szg61bw
The Streamliner is my weapon of choice, It's a great amp and weighs next to nothing.