Just do some statistics on your current setup. I am asuming you are using the 6500 for you servers, so turn on netflow and gather the data.
Having 10Mb at the desktop in a Microsoft enviroment is generally a bad idead. Let's assume you have a Windows 2000 file server, An Exchange server, Internet Web, Network Printers.
The Exchange RPC packets are quite large, and most people have Outlook running all day. The File server is probably housing a fat Excel spreadsheet, or a 100mb powerpoint. Maybe just pr0n though...
If a user has only a 10mbit connection, that means they can only transfer data at 1.2Mbytes per second. If they have 100mbit they can transfer at 12Mbytes per second. Meaning a 100 Mbyte Powerpoint will take 83 seconds to save or open on a 10mb connection, but 8.3 seconds and a 100mb connection. Drastic increase in performance there. Taking 1 1/2 minutes to just open or save that file is a cut in employee productivity.
You also have to look at your server load. At any given time, how many of the 3000 users are copying/writing large data to them? If the server's NICs can't keep up, you should be able to throw down $100 for a 1gbit NIC card to increase performance. All of your switches mentioned support this technology.
If cost is an issue (hard to beleive considering having 5509/5513, 4506, 4006, and 6500's), then you can do rate limiting on your vlans/segments so the user's vlans/segments don't suck all the bandwidth.that you have quite a bit of