Based on information I can find, your system has a PM965 North Bridge and supports Socket P Intel chips. These would include Penryn-core Core 2 Duo with an 800 MHz front side bus speed (up to a Core 2 Duo T9500).
This is the same basic hardware as my DV9700t.
Depending on if the chip is soldered to the motherboard or if it is in a socket, you may be able to upgrae it. Whether or not you NEED to depends on the chip currently installed (basically, if it's a T8300, T9300, or T9500, I wouldn't bother).
As far as RAM, you have a max of 8 GB (the info I have says you have two slots, each should support a 4 GB stick).
As far as hard drive capacity, you can put basically any 2.5" HDD in there. I have a 320 GB HDD and a 128 GB SSD in my system. There are 500 GB and some 750 GB drives out there. There ARE 1 TB 2.5" HDDs but so far all the ones I've seen are non-standard height and won't fit in many drive bays.
The system would have no issues running a 64-bit OS (provided there are drivers available, though this would only really be an issue with an OS other than Windows).
The only real restriction in your system will be the video. It would be worth your time to decide if the cost of any upgrades is worth being stuck with whatever video you have.
Of course, your mileage may vary, but I have done similar upgrades on other systems at work with the same basic hardware and been successful.
O Shadow Fox O