Sure here's my opinion.

Raid 5 is probably fine for 20-25 users. That being said it is the worse performing RAID choice (in terms of writing to disk) but cheapest in cost. If that's your only choice go with it. We have clients running similar size systems with raid 5 and it's not too bad. That's my opinion for your case but what those guys said about it is correct but's better than a single disk system.
Memory is very important!
If you have the chance use RAID 0.

That's just my opinion.

For detailed info Check out.
http://www.ipass.net/~davesisk/oont_performance_raid.htm

(A brilliant link on the subject I have refering to for a while)

Cheers.