Yes, get at least 24 GB of RAM and at least one Hard Drive would be my minimum suggestion Thirdly get a GTX 560/570 class graphics board
To save money do not order from HP
I concur with Bill's priorities, RAM, disk, video and his choices, but I would say at least TWO HDD's.
Incidentally, you do not have 4 CPU's the maximum for this system and with Windows is 2 CPU's. They are quad core units. Do you have both sockets used?
Another possibility would be to sell the HP and build a new system. My gut feel is that your not too terribly old dual Xeon HP workstation with Quadra 1800 is worth more to users that are not running Premiere Pro CS5/5.5. And, while core i7 memory is super affordable now (for a new quad or six-core build), I suspect that adding 24GB of ECC RAM to your HP would be expensive.