Hey Mark,
I'll throw in my 2 cents. To second what Neil said, you should be able to get SWOP like proofs fairly easily. All adobe apps have this functionality built in. Choosing a proofing paper that is close to the white point of your desired proofing environment will help. The contract proofing paper satin is quite good and use few optical brighteners. Keep in mind too that the SWOP_V2 standard has been updated with with two new specs for printing on a #3 and #5 sheet, as well as the GRACoL spec for a # 1 sheet. Relative colorimetric rendering should give you good results, but if your proofing to a SWOP_V2 or SWOP2006_#5 spec the white point of the proofing paper will probably be quite different so you may get more accurate results using the Absolute Colorimetric rendering. If you looking to deliver contract quality proofs you can probably get there without a RIP, but not easily.
A RIP is often perceived as offering better quality, what it really offers is accuracy and workflow enhancements. A good RIP will restrict ink to optimize the printer gamut to that of the proofing environment, and then linearize that output. Rips will often give you a simple way of optimizing and verifying the prints you produce, as well as offering better postscript support. They will also usually give you a much easier path to making that print, such as using hotfolders or having multiple workflows for different proofing conditions. The EFI RIP as well as many others interfaces with the on-board spectro to automate the linearization, profiling, optimization, and verification process. So depending on the quality and quantity of prints you want to make a RIP may be a life saver. Generally I tell people to try the option they have available first and the go from there. Let me know if you have any other questions I've worked with a lot of RIP's and the Z3100.
