Frederic,
All other factors being equal (and provided sufficient RAM, which you don’t have) a CoreDuoQuad will render approximately twice as fast as a Core2Duo.
When I switched from Vegas Pro x32 to Vegas Pro x64 just recently, my render times for a 10 minute video with maybe three or four layers dropped from two and a half hours to maybe fifty minutes (more than twice as fast).
I don’t know your budget, but you could upgrade cheaply by getting a CoreDuoQuad and 8GB RAM, and then format C:\ and install Windows 7 x64 and Vegas x64. That will be a budget upgrade that will make your rendering maybe (I’m guessing) three or four times as fast.
If you have the funds, a Core i7 is supposed to be at least 40% faster than said CoreDuoQuad (above) before any overclocking (and overclocking will take it even more, provided you have sufficient cooling). So maybe five or six times as fast.
Also please bear in mind that you can get great performance gains by having more than one drive. Leave Windows and Vegas on C:\ (minimum 7200 RPM spin) and put all of your project files on D:\ (minimum 7200 RPM spin but some say a RAID 0 or RAID 10 configuration will move even faster).
I hope that helps.
Norman Willis
http://www.nazareneisrael.org