If you can get a ~$100-$150 N800 and paying ~$500 for the next tablet when it comes out, then I'd do it. The N800 will be a lot of fun in the mean time, and it'll get a big breath of life once Mer gets day-to-day usable. Then you can buy the next one when it comes out.

If ~$500 in addition sounds like a problem to you, then maybe buy the N800 anyway and wait until the next tablet's price comes down a bit and buy it then.

If that sounds like too much time with old hardware, then just wait for the next tablet.