I've heard PC users talk about their on-board RAID and overhead, not Apple software RAID that I've ever seen. And the Apple RAID card isn't worth it. And better ways to spend $$ to boost performance. Also, internal controllers are few and not worth the trouble usually.
Negate the performance impact? My suggestion is to do some testing and experimenting, get your feet wet, and get a better feel for, and understanding, of what you can do, and what to expect. And invest is drives that are a good match for RAID.
As for boosting performance and such, there are a couple ways to go. I keep all my data on a stripe RAID separate from the boot drive that has just OS and Apps.
SSD is fast and instantaneous loading of files.
10K WD VelociRaptor is what I am using for system on three computers, and for data on one.
Samsung F3 1TB looks to be one of the tops today in performance.
Memory. I've posted a number of links to articles - today. But 6 x 2GB is what I would call a good base number. More memory only helps if you or CS4 etc NEED it and will take advantage, say you work with 1GB and larger files in Photoshop and lots of layers.
http://eshop.macsales.com/Reviews/Benchmarks/NehalemTests.html
http://www.barefeats.com/nehal04.html
A lot depends on your files, work, etc.
http://www.macgurus.com/guides/storageaccelguide.php
http://www.macgurus.com/guides/photoshopguide.php
http://macperformanceguide.com/
CS4:
http://kb2.adobe.com/cps/404/kb404440.html
And upgrading to Snow Leopard, but only when you, and your system and programs, are ready, also will pay dividends.
