There's no need for a RIP or a much more expensive printer for soft proofing. The key requirement for soft proofing is a calibrated and profiled monitor - without an accurate monitor profile, soft proofing isn't going to produce workable results.
If you have a recent version of Photoshop, that will allow you to soft proof and adjust the images if necessary. I've done this in Photoshop CS2 by adding some adjustment layers to the PSD - if I need to do it again, I'll use exactly the same techniques in Photoshop CS3 Extended.
Hopefully Lightroom will get soft proofing features in a future version - as usually all it needs is a slight change to the curve (or similar tweak) to get the tonality right.
Meanwhile, if you were on Windows, you could download a trial version of Qimage, and see if that does what you want. I don't know what Mac users tend to use instead of Qimage. Maybe the Printfab RIP you mention is the best solution; it's not as expensive as some of the RIPs on the market.
There is a print preview function in the B9180 driver (at least there is on Windows XP), but I'm pretty sure it's for layout purposes only, and isn't a soft proof.