I've found Confio Ignite (free version here: http://www.ignitefree.com/) very useful for benchmarking database servers. You can monitor type and quantity of wait states, and get a very good picture of what resources are being overutilized, underutilized, or are just fine (all 3 bears) on each server.

If, for example, every server is suffering from a lot of CPU/RAM waits, then consolidating onto common hardware is unlikely to be successful, because they'll all add together and be even worse.

On the other hand, a server with medium-high CPU waits, but with low I/O demands, might consolidate well with a server with medium-high I/O and not a lot of CPU, if the consolidated server has improved CPU + RAM and improved I/O, over each of the servers that are being consolidated. They won't collide with each other, in other words.

Best of all, is if you have a whole lot of servers with a lot of idle time on all of them. Add up the CPU waits and find that it's less than a single server can handle, add up the I/O demands and if a dual-channel fabric to a good SAN could take all of it easily, and you have a great opportunity for consolidation.

Ignite, or RedGate's SQL Monitor, or SQL Foglight/Spotlight by Quest, will help you find the numbers you are running on your current servers. I/O bottlenecks, CPU bottlenecks, network latency issues, and so on, will all be measurable with any of those.