I find no performance differences between Vista and XP. That is, running Vista on 1GB with a dual-core 1.6GHz and running XP on 512MB of RAM with a 1.7GHz single-core Pentium 4.
To be honest, no you probably wouldn't find a differnce at that because the dual core would be a considerably more powerful machine and with double the memory, vista struggles with 512 mb