In Vegas Video more RAM improves the RAM preview, which "speeds up" rendering (you can now RAM preview more video). If your projects are really short you might be able to RAM preview your entire project. I don't expect that more RAM speeds up rendering.
This site doesn't find more RAM helpful in speeding up render times. If you mess around with the BIOS (pretty safe nowadays) you can take advantage of quality RAM and speed up rendering times.
CPU is a huge factor in rendering times and many real world benchmarks reflect this. A faster front side bus also helps. Pentiums are also better at encoding for certain programs (most MPEG2 encoders) as I already mentioned.