Welcome back to the good side my friend
As far as what chip to get, I either go with the new p3 tulatin (sp?) or the new P4 Northwoods. Don't bother with the older P4's (Williamette's) cause they have a smaller cache, and are only a couple dollars cheaper. If I were you, obviously this is biased a little, I would get the new P4 1.6a or 1.8a ($129 and $183 in that order). The 'a' on the end denotes that it is a northwood. If money is no issue to you, I would recommend getting either a 2.0 or 2.2. The prices for a 2.0 run about 260 and a 2.2 runs around 380. All the prices are off of newegg.com. Any way you go, however, you will be able to overclock the heck out of these chips. The 1.6a can overclock as high as 2.7! While the 2.0 or 2.2 can't get that high a percentage of an overclock, they can overclock really high as well. I have my 2.0a at 2.72 with full load temps not going over 42 celcius.
I would offer you some advice on the newer P3's, but I don't really know too much about them, besides you can get a pretty decent overclock. Maybe you can do a search for some threads on this, or someone who knows somthing about them can chime in and help you out.
As far as motherboards, I'll give you the RDRam side, as once again, I am not really familiar with DDR. Don't be scared away by the bad things you always here from DDR fans about Rambus though. At the current moment, you can pick up 128 megs of RDRam for as low at $29 on pricewatch (although you have to be careful what ram you get...if you have questions just post and I'll help out) and 256 chips can get had for 66. The one thing about the RDRam boards is that you must have TWO sticks of Rambus, as they work in pairs. If you want 256 megs total ram, get two 128 sticks. If you want 512 megs TOTAL ram, bet TWO 256 sticks.
Anyways, back to the boards. There are two main RDRam boards everyone is using. The Abit THII (which offers a raid controller) and the Asus P4T-E. Both have their ups and downs, but both are really really good boards. I have the P4T-E and LOVE it. I haven't had one problem yet.
Hope this helps! Glad the AMD Demons finally release you