yapadu,
you must remember that the number of domains won't necessarily mean a lot of emails. we run SF with over 4,500 domains. one primary, one secondary (running on VMWare). granted most of these domains are dormant, or don't even receive mail, but due to the nature of our setup we can't tell who does, and who doesn't (we're in the reseller market).

Here are some stats since about Jan 2007 for our primary MX:
total inbound connections: 206,081,880
email forwarded: 69,575,871
emails blocked: 81,122,647
email attempts: 133,759,396

during the day we have an average of 50 concurrent connections.
these are stats for secondary MX which get much more spam. these are from about Jun 2007
total inbound connections: 252,058,935
email forwarded: 40,661,683
emails blocked: 88,951,851
email attempts: 120,553,366

We had instances were we refused service to some customers, simply due to their email volume, and their refusal to pay more for the service.

i don't think SF has any problems with the number of domains. you should be looking at the number of concurrent connections you're going to run.
although i must say that ever since the implementation of the grey filtering, we've seen a reduction is resources used on the servers.

good luck
Amir

you'll quickly realize that the "little" machine if setup as a secondary will be processing MUCH more spam then the primary.
just the way spammers work.