I'm running a 2009 iMac i7 Quad HT ith 8GB RAM, using Premiere and Photoshop CS5.
It's very fast. Faster than the MacPro I use in the office (with 32 bit apps) the more RAM then in theory the more speed, but in actual fact you really need to consider the speed of the drives.
And you will need more than one.
Ideally you want a drive to run the OS and apps, a capture scratch (for your RAWs, and H264s) and a render or output scratch to write to.
This is going to get the most out of your machine.
THe second thing to consider is getting an SSD drive for the system drive (gets data in and out very quickly) and look at the connections the mac mini supports.
There isn't much scope for internal expansion with a mini, so you are looking at an external option. A fast RAID can be set up with great capacity for relatively modest cost. Something like 2x E-Sata II or III Seagate Barracudas in an external RAID caddy with the drive striped using software raid will give you SSD speed but with TB's of space.
If you could spec two RAIDs, one for capture and one for render then your Mac Mini would absolutely fly.
However do note that a Mac Pro tower has internal bays for expansion, so if you need speed this may be the best bet (put an SSD in the spare optical bay, use the 4x 3.5 bays for 2x software RAIDs)
Not to mention the extra grunt at the processing.
But RAM is good. 64bit is good. The thing that slows most folk down are their drives. Usually the last thing they upgrade. You want multiple. You want speed.