Lightroom 2 is feature-matched with Camera Raw CS4. CS3 is feature-matched with Lightroom 1, so if you're processing RAW files with CS3 or earlier, you are missing out on some features (though, admittedly, this may not affect your own workflow to any great extent).
As others have stated, the main advantage to using LR2 in addition to PS CS4 is to gain a great RAW management tool. The fact that you get RAW editing tools on par with Camera Raw is a bonus if you have LR.
So the short answer is you don't need both if all you want to do is edit RAW files. If you want a nifty RAW management tool as well, LR2 + PS CS4 is a great combination (and if you happen to know a student with an educational discount, you can get both for an absolute steal!)
I personally use both; LR2 for management and most editing; PS CS3 mainly for layers, perspective correction, sharpening, and adding borders and watermarks.