It'd be very cool if you could get one of these things to run in your G5. But thinking realistically, is it really worth the tremendous cost for anything past bragging rights?
For one, I agree (as a non-expert, admittedly) that once the majority of the OS is preloaded into memory, there wouldn't be a huge speed hit. And if you look at the way OSX allocates App memory, it caches quite a bit of what an app needs (even after you quit it) in RAM, so after the first launch there's not much hit there.
But forget that, compare one of these RAM drives to the alternatives:
The base 2GB model will cost $700, + about $250 for the RAM to put in it. That's just shy of $1000 for 2GB of storage. Go up to 4GB, and you're going to spend at least $1400.
Compare that to the Fujitsu MAS3184 15,000RPM SCSI Ultra320 18GB drive. About $200 at NewEgg. Assume you will be able to buy an Adaptec Ultra320 hardware RAID PCI-X adapter in the $300 range.
Making those assumptions, for a little more than $1000 you can have a 72GB, 4-drive, RAID-0 15K RPM array. Being that the individual drives have latencies in the 6ms range, that's what you'd call fast. Definitely not as fast as a RAM disk, and it'd make one heckuva lot more noise, but I have a feeling that'd launch an app pretty darned fast, and you'd have 36 times more space to play with.
For the same price as the 4GB model, you could get yourself two MAS3735 73GB 15K drives instead. That's 140GB, with a hardware transfer rate of 150MB/s in a two drive array.
Sorry, if I had that kind of money to blow, I'd go with a beastly SCSI array or an XServe RAID. Since I don't I'll just be very happy with a $150 WD SATA Raptor in a G5.