Upgrading my rig

Hello allll.

I'm in the process of upgrading my custom machine that I 6 months ago for mostly fun.

It currently sports:

2x2gb DDR2 800 ram
Nvidia GT 240
Intel e6300 dual core 2.8ghz (the newer one)
Asus p5kpl-cm mobo
500W power supply that came with my antec (bronze?) case
and 500GB HD with 1TB external hd

If you guys could tell me what to upgrade to in terms of individual parts or the entire machine I would greatly appreciate it. I also don't want to spend too much money because I'm starting my first year of uni next month and need about $25,000 ^_^.

what do you want to do with this system?

Like I said, I would like to upgrade this pc, or if you think that its not worth it, build a completely new one. I want to use it for playing SC2 on ultra at 1080p, along with running engineering applications for school.

I also have another question:

I currently have a macbook pro (2.66ghz dual core 9600GT 15.4" & 4gb ram) and would like to know if I should also bring a desktop to my dorm @ university or just bring the laptop. Thanks.