My boss just got a r50p with 1GB - they actually SEND you the SO-DIMM and tell you to install it! There is only ONE slot, too, so that's it.
I believe it's just PC2700 SO-DIMM... It's not a fast machine, for high-end and gaming. I definately would NOT look to a Centrino/Pentium-M solution for that. It's great on battery life, and decent on normal performance.
Get a nice P4 or AMD64bit solution for the high-end stuff. Battery life can take the hit.