Incremental upgrades such as this are pointless - you lose a lot of money for not much of an upgrade.
The new iBooks appear to clock in at 1.067 GHz, whilst the rev B PowerBook 12" (my model) has a 1.0 GHz CPU - both have 512k L2 cache and 133MHz FSB.
If you upgrade to the PowerBook, you'll see a difference, not to mention all the other perks: optional Superdrive, audio in, mini-DVI out and native monitor spanning, better keyboard, AirPort Extreme card built-in, built-in Bluetooth, 64MB graphics, optional 5200 80GB HD, etc. But it's a lot of money when your current computer still does things really well.
You may as well stick it out with the 800MHz iBook for a few years, then get a new G5 12" PowerBook when it eventually arrives. There's absolutely no point whatsoever in getting the new 1GHz iBook.
ps I don't trust xBench. My PB gets 95-100 score but it is widely variable even when doing the same stuff.