Posted at another forum:

While I'm on vacation this week I took some time to install Vegas on my 64bit system. I've installed both 8.0c and 8.1 versions of Vegas and everything seemed to be going UNTIL I attempted to compare render times on my old 32bit system vs my new 64bit systerm. I took a 1 minute / 10 second HDV clip and placed it on both machines and attempted to render it into a MainConcept AVC [mp4] file. Launched the render on my 32bit system without a hitch as usual but when I went to render it on my 64bit system under 8.1 I kept getting the system memory low error. I went into the system settings and entered custom amounts based on the 8GB of available RAM but I still keep getting system memory low errors. The plot twist to this is that I can render into any other format execept MainConcept AVC and Quicktime mov formats with no problems.

Any other 64bit/8.1 users ever experience this issue?

The only place I have both installed is on my laptop, which is grossly underpowered and short for RAM. I think your problem lies elsewhere. Open up task manager and see what is eating up your memory.

I just got an email from Sony support stating the following:

"Thank you for contacting Sony Creative Software. After consulting with the development team on this incident, I have been informed that the MainConcept AVC mp4 file format under 8.1 is causing some unexpected behavior and has been listed as a bug at this time. This issue should be addressed in a future update to the software. For the time being, I have been informed that the current work around would be to use the Vegas Pro 8.0c to render to that specific codec."

I installed Quicktime and that eliminated my problem with rendering to the .mov file format

Since you have both version of vegas on a 64bit OS with more than 2.5GB of RAM, have you noticed if the 32bit version of vegas is able to use more than 2.5GB of RAM?

I'm still shooting SD MiniDV with no plugins, so I probably don't have to worry about using 8.1 (seeing as that is practically the most vanilla of all video formats). But I worry about installing 8.1 on the power edge and opening files created with 8.0c on my 32bit edit system. If I change anythign on the timeline using the 8.1 system, will my project file be available in 8.0c? or will modifying the file in 8.1 "upgrade" the file in a non-backwards compatible way?

Since you have both version of vegas on a 64bit OS with more than 2.5GB of RAM, have you noticed if the 32bit version of vegas is able to use more than 2.5GB of RAM?

I'm still shooting SD MiniDV with no plugins, so I probably don't have to worry about using 8.1 (seeing as that is practically the most vanilla of all video formats). But I worry about installing 8.1 on the power edge and opening files created with 8.0c on my 32bit edit system. If I change anythign on the timeline using the 8.1 system, will my project file be available in 8.0c? or will modifying the file in 8.1 "upgrade" the file in a non-backwards compatible way?

I open the same .veg files in 8.0 and 8.1 all the time.