Not so. When it came out, RDRAM was the answer to the problems DDR hadn't overcome yet. It was, however, far more expensive to make. However, DDR soon hit its stride and surpassed RDRAM. So modern DDR (400MHz and up) out performs RDRAM. DDR2 is even better.
You won't find RDRAM hardly anywhere, because they stopped making it years ago. I know. I tried to find some for my sister's PC. Ended up just buying her a new motherboard instead.
As for how well it'll run...
P4 2.66.
768MB of pc800 RDRAM (booo).
nVidia Ti4600 card w/ 128MB
It just so happens I upgraded from a Ti4400 not long ago. My CPU is about the same, but I've got DDR ram (and more of it). I used to have just 512MB, then got a stick of 1024 (1.5GB total) then donated the 512 stick to a good cause (1GB total). You will have to stick to low texture resolutions. Default might be good for you (256x256). You need to run AH at least at 1024 resolution, but it's possible on that card. Keep your detail sliders at default ranges, or even reduce them. You should have a steady 30-40fps (in best conditions), dropping to 20fps or less in heavy smoke/fire and large furballs near CVs.
It's playable. That's for sure. I won't go back to that hardware setup, but it ran. It should be enough to get you addicted to the game
