sorta. I use reason 3.0 on a 600mhz machine with the same amount of ram. You don't even need the better soundcard, as long as you can hear what you are doing, thats enough, as once you burn your songs to disc, you have bypassed a soundcard altogether. For my monitors, at first I just used a really good pair of headphones, but ended up buying a thrift-store marantz 220r reciever (10$) and a pair of 1980 kenwood 3-way floor speakers (free). This was my setup for many years until I upgraded to a real pre-amp,power amp, and mid-70's JBL speaker set up. I still use an absolute garbage soundblaster card(24$) to record hardware synths(ack!) I have an Audiowerk card coming soon to fix that.

I think your computer is plenty kick-ass. More ram is a start.
Monitors are the least of your worries.
The roland gear you mention is not hardware. They are computers, so if you can get a digital output on them, you can bypass the analog stage and record straight to computer. That way may save your wallet.

GAZDATRONIK