The A608 will be a great upgrade when they're finally available, but it could be quite some time before a second batch of them are made, and the first batch (which are already preordered/taken) is only just going into production now.
A 2MB SRAM card will in fact speed up your A600 very slightly, although it will slow an A1200 down. Here are some SysInfo results from a test I did:
A600 (2MB Chip, No FastRAM)
526 Dhrystones
0.54 MIPS
A600 (2MB Chip, 2MB PCMCIA SRAM)
699 Dhrystones
0.72 MIPS
Upgrading my A600 to a total of 4MB RAM has allowed me to play hundreds of WHDLoad games which wouldn't work before, as well as giving me more memory to work with when I have multiple applications running. I think they are definitely worth getting if you have an A600 and you don't want to wait possibly another year for a chance to grab an A608. But $100 is a lot! How about $39.98? That softens the blow a little doesn't it? You can buy them from Memory Ten, it's where I got mine from.
http://www.memoryx.net/bgz.htmlI just hope nobody goes and buys them all up to resell, this is the cheapest place I've found in the world to get them and I think all A600 owners should have the chance to get one at a good price.