Most people use the "Byte Unix Benchmark" system to do this - there's actually a thread over here where people compare their results on a linux vps - there's one for dedicated servers too but i can't find it right now.
Byte is very common, here are some others for your knowledge:
LMBench: Suite of simple, portable benchmarks, useful for comparing performance of different UNIX systems
Ubench: A simple cpu and memory benchmark for various flavors of Unix
Phoronix Test Suite: open-source benchmarking suite for Linux, OpenSolaris, and FreeBSD
To bench your web server, you can simply use "ab" (apache bench) that is available with the httpd package in CentOS, and apache2-utils in Debian. It works quite well...
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