Actually, the PS3 has 256MB of vRAM on the RSX and 256MB of XDR system RAM. It can also pool up to 224MB of system RAM, shared from the CPU (total of 480MB for video). The 360 on the other hand has a dedicated 512MB of GDDR3 system RAM and 512MB on the Xenos GPU.

Almost, but not quite....
The PlayStation 3 has 256 MB of XDR DRAM main memory and 256 MB of GDDR3 video memory for the RSX, yes, that's right, but...
The Xbox 360 pools all its resources from 512MB GDDR3, that both the system AND the GPU share. Shared memory. The Xbox and the PS3 both have 512 MB's of RAM/VRAM.

Effectively, they basically have the same amount of RAM and VRAM. Streaming technologies are used to get over RAM limitations as well, as seen in Killzone 2 which consistently featured 1024x1024 environment textures and even 1024x1024 POM ground textures, as well as POM decals from bullet holes.