Depends how serious you are about keys. I bought my first 88 fully weighted 4yrs back. and would now have nothing other than 88 fw.
Just depends what works for you.
I have 3 sets of keys now (takes up a fair wack of space) did have 4 but that was a nightmare unplug one to plug the other in.
Id recommend unless you play live or like to jam 2 boards at once.
Either buying a synth with loads of controllers forget the midi board unless its portabilty your after. Or the midi controller with sound modules take up less space unless you would like to jam both the midi controller and synth together.
If your not after jamming both i would recommend much more powerfull synths than the microkorg i had the ms2000 (more powerful than Micro) but sold it (i love korg but can't justify this synth over things like novas virus radius clavia waldorf etc etc).
Any synth will fit into your setup but seriously go play with them before you buy.
if you don't know there sites heres some links to get you started. also read there manuals or quick start guides find there features that suit you.
Micro
http://korg.com/gear/info.asp?a_prod_no ... egory_id=1
Virus
http://www.access-music.de/audiodemos/
Waldorf
http://www.sequencer.de/waldorf/waldorf ... sizer.html
clavia
http://www.clavia.se/MP3s/index.htm
On ebay you can get some stunning bargains on these synths.
My ms2000 (more power than microkorg) cost £150 microkorg at the time was approx £180 sh