I have both.
P4P is an Intel i865 chipset, has SATA+PATA, crappy onboard video, 4 DIMM
slots (dual channel) all 4 slots PC3200 max.
P4R has ATI 9100IGP chipset, PATA, best onboard 8.1DirectX hardware video, 4
DIMM slots (dual channel) 4 slots max in PC2700 mode, 2 slots max if PC3200
mode.
This P4R also has Qfan built into the BIOS. Alows you to control 2 case fans
speeds by BIOS, a nice free extra bonus.
If you want full power, most options, then Intel chipset (add better video
card).
If you want to use all onboard features only, then the ATI 9100IGP.
I run both with 2Gb of PC2700 (4x512 sticks) in dual channel mode. The P4P
with a 3GHz P4 HT - the P4R with 2.8GHz P4 HT. They are identical in
performance.
You can make the P4R nearly completely silent, use a Zalman 7000a AlCu
heatsink for the CPU (in 1400rpm silent mode), and a Zalman chipset heatsink
ZM-NB47J on the video/northbridge.
With a good power supply, you wont know it's on.
http://www.zalmanusa.com/usa/usa_index.asp