7200rpm drives are mandatory for good audio work especially with pro tools and most imacs of the last generation didn;t come with them. that is a huge thing that will slow your pt sessions down, and its not a mac issue, same thing with pc's.
also 1.5 gigs isn't much anymore. i just upgraded from 1 to 3 gigs on my macbook pro last week and it feels like a new computer. if you're going to get a whole new comp nevermind but i'd really suggest you fix those 2 things if you're needing more speed on the imac.
wow... I mean... f_ckin WOW!!!
Y'all must not have been around for too long. You can comfortably fly 24 tracks of audio off of a 5400RPM drive as long as the interface you're using is fast enough. To give you an example, my first serious DAW machine was a Pentium 233MMX machine with 128MB RAM, Win95b (later upgraded to Win98SE), Adaptec 1520 SCSI card, pair of Seagate 4.3GB SCSI-2 hard drives, Pinnacle 4x external SCSI CDR drive (that's burn only y'all, no RW), Event/Echo Darla, and Cubase VST 3.7. Even though I had other machines before this one, I consider this to be the first serious one 'cause it had everything you needed to fly 24 tracks off ya PC.
And that's what it could do. 24 tracks of audio without a hiccup. Couldn't use much in the way of fx back then 'cause I was still on a Pentium MMX and not a P2 but it didn't matter much 'cause I was running a hardware-based studio and had a rack full of fx.
So... 7200 RPM ISN'T mandatory, it's just nice to have. What matters more is your sustained data rate; not RPM speed. My old SCSI disks had great sustained data rates and these days even an old ATA100 drive will too.
As for 1.5GB not being enough, I scoff at you. My main DAW (Athlon XP 2000+) has 1GB of RAM, what WAS my second DAW(P3-850/100) has 512MB RAM (I just gave it to my wife to replace her old PC). My current backup DAW, known as Three-66 for it's Celeron 366, has 256MB of RAM.