Thanks. Ok, I will do some testing this week end.
Can you recommend me any solid performance test utilities for linux guests?

At least, less configurations to test for me.
If No.4 can not be run without VT, then I suppose that No.8 neither.

Thank you all for help. I understood that it makes no sense to switch VT support off, so I compared 32bit and 64bit distributions of CentOS 5.4.

I used Phoronix test suite and the host was running ESXi 4 on IBM System x3500-7797 (Xeon 5405).

I run a few tests (build-php, postmark, sqlite, pgbench, openssl, memory benchmarks and some more). The outcome is that 64bit distribution is a bit faster.
The only noticeable difference was openssl benchmark. 64bit was double faster.

This leads me to another question, is there any reason to choose 32bit guest over 64bit or vice versa for standard services like DNS, MAIL, WEB, maybe MySQL?
Of course, I suppose that guest has maximum 4GB of RAM and I am planning to run only the free ESXi.

One more question to my host setup. IBM System x3500-7797 (Xeon 5405) is not listed on Hardware Compatibility List for ESXi.
I have run it for one week without any problems. The health status is all in green. Where can I look for any incompatibility clues (logs, or anything)?