The only catch with the HP quad cores (or any quad) might be that your production software isn't multi-threaded and so will run SLOWER on a quad core than a faster clocked dual core.
Is there a forum or website where you can research your production software, find out of it's multi-thread for quad core processing?
Since you're out to earn money with this laptop and you don't have another computer to help with the workload it seems to make sense to upgrade the laptop a bit. This is especially true if you software is in fact multi-threaded and will get a good boost from a quad core CPU.
I don't know if T9600 -> T9900 (+.26Ghz) upgrade is worth $300. My guess would be no. Passmark CPU benchmark scores
: (remember - useful comparison for multi-threaded software)
T9600 = 1984
T9900 = 2358
C2Q 9100 = 3391 (2.26Ghz x 4 cores)
Core i7 720M = 3356 (1.6Ghz x 4 cores)