If its a laptop, you'll be expecting a late G3 or an early to mid G4 iBook, I put my money on a G4. Assuming it hasn't been modified heavily, you'll have something between a 900mhz to 1.25ghz (or maybe 1.5ghz but I'm not sure if they go any faster) PowerPC processor, 128mb to 512mb ram (possibly higher but not likely, you certainly won't be seeing 1GB or more if it hasn't been modified) and a 40gb to 60gb hard drive. You'll be running Tiger probably (or if its a G3 Panther).
I'd be surprised if it doesn't fall around those specs. As for Blockland, I wouldn't get too excited. The lower specs, PowerPC processor, and graphics card will probably mean you can join servers on medium to low graphical options with smallish brick counts. Aside from Blockland, you should be able to browse the Internet nicely, create some cool things with the earlier iLife versions (including iMovie 6 HD, the tool I use for movie creation still :3) and run a lot of nice PowerPC programs. Not bad, as long as you don't want to run the latest and greatest programs and games, and don't mind a bit of lag on BL servers.