Someone like Stefan could answer this question with much more expertise than me, but I would say the 512Mb would definitely clinch it! That's a helluva lot of VRAM. Won't necessarily do much for your rendering times, but the 3D Window (OpenGL) will just fly. I have 256Mb on my laptop (Radeon X600) and it spins complex buildings faster than a disc jockey on speed.
Cheers,
Link.