Laura made a great post so there's nothing much to say here...
This is your problem... If you need a keyboard just for fooling around
and you just want to give a try on keyboards then go for the cheep
stuff. PSRe403 will do just fine...
BUT if you wanna make something out of your music then i really suggest
you wait and save your money for the real thing.
I had NO idea how much enjoyable is to play on semi-weighted keys
(not to mention the full-weighted). Now when i have this kind of keyboard
myself, i really can't (and won't) play anything with plasticky keys.
I've passed a lot of keyboards (PSR185, PSR340, PSR2000, KORG i3,
KORG TR, MOTIF ES, some of the Clavinova series) so I know what I'm talking about.
Don't rush into things and buy a cheep keyboard. That way you won't be
able to buy anything else for a while (cause you've spent your money on
some cheep thingy).
There are some good 88keys keyboards for a low price out there.
They're not as good as Clavinovas but they're waaaaaaaaay better
then any of the PSR series.
For example:
M-Audio ProKeys 88,
Yamaha P70,
Yamaha P85 (around 600$)
I know this is out of your price range, but it will be worth waiting.
Don't eve bother looking at pro workstations cause they cost over 2000-3000$, and you would be using 10% of their capability.
And the reason why pro players don't like the MM6 is cause it "has" MOTIF sounds but stripped to the bone, and polyphony is THE most important
thing in keyboards. Just next to the sound quality and key type.