Depending on how much the increase in speed is worth for spending money:
You have the MacMini2,1. Max out its RAM at 3GB and stay with Snow Leopard and FileMaker Pro 8 and consider a SSD.
I'd get the 2.3GHz i7 Mac Mini and max out the RAM and possibly consider replacing the hard drive with a SSD and upgrade to FileMaker Pro 12.
Check out RAM upgrades & SSD's at OWC
Just for curiousity:
Here is how I run FileMaker 7 in Snow Leopard Server installed into Parallels to run in Lion (or Mt. Lion):
[click on image to enlarge]
I do not use it often enough to pay for the upgrade.
...Is there any way to tell whether the next new computer is just around the corner?
That's always the tough issue; they keep these things pretty close to the vest.
You can google some "rumor" sites to see what may be brewing, but quite honestly even these sites can be way off.
At the end of the day, the day you need a new computer, is the day to buy. I always buy as much hardware power as I can afford and put the money into software updates later.
I lived with my iMac G5 desktop until August 2011 (when I purchased my 2.3 GHz i5 Mac Mini) and my Powerbook G4 12" until the first 13" MacBook Pro came out in 2009. And those are what I live with now (with the addition of two Snow Leopard Mac Minis used for Home entertainment and other purposes).