[support] Two versions on same server??

[Prothero William]

Folks:
I have a drupal 6 site on my server, but want to eventually upgrade to version 7. I have one of the bottom of the line VPS servers at liquidweb.com, so am a bit concerned about the server load. For multiple sites, the sites folder holds the configuration information for each site, but if I use D7 in another domain, would this cause a big impact on my server? 

Basically, I'm asking if this is a stupid idea, and what tradeoffs, re resources, I should consider.
Thanks,
Bill

Prothero William
waprothero at gmail.com