If you're worried about downtime being too long, I'd go with the Opteron. The HDD choice though might be a problem; I think you should go with an SCSI drive with at least 10k RPM for the main drive, as it will allow more I/O, though the space on it will be smaller and will cost more than a SATA drive.
Can you explain in layman's terms?
I'm honestly not worried about having the downtime, if it means me being on a better server. 8 hours is nothing, in the grand scheme of things. So, if the downtime would be worth having a better setup, that is totally fine by me.
I did check out the SCSI drive upgrade and it is an additional $120 a month. You say it helps with I/O, does that mean In/Out? As in traffic?
And, honestly, the space isn't a big issue for us right now at all. We are FAR from outgrowing the space on our current, lower end server. It's mainly just database resources that are killing us, because I've got the two forums running on one server.
Eventually, I know I will need to house each forum on their own server, but one of my sites is still so small right now, I don't have an immediate need for it.
The person I spoke with at my hosting company said that the bench marks for the xeon and operton are nearly the same, with very negligible differences, but xeon tends to just be marketed better... kind of like Nike versus Rebok or something. I just need some second opinions from people who are familiar with how vbulletin forums tend to run on different types of servers.