Thank you for your detailed reply. I would just need some clarification:

1. How should I determine the existence of a VCPU bottleneck? At guest level or ESXi level? Any suggestions/tools/resources? Also, an important question: HOW should I increase VCPUs? Increase cores or processors?

2. I have to admit I haven't thought the RDM solution. Seems nice. But I am not sure what you mean by a virtual RDM. Also, why can't I snapshot a normal RDM?

3. What exactly is 'thin provisioning'?

4. So far, I am doing incremental backups at guest OS level, using Acronis 9.7 agents. In my test ESXi environment, I have tried Backup Exec 2010 R3 as well as Acronis 11 Virtual. In my opinion, they are both problematic.I haven't been able to do a satisfying backup/restore under any scenario. Very slow, buggy. Apart from that, I do not have so many guests to having to backup at ESXi level. I will most probably continue doing the backup at guest level. In my 2TB guest -a mail server as you might have guessed- I will soon be switching to maildir format, so I guess the incremental backup will be considerably faster. So I am seeking the best solution for backing up guest OSs directly.