Starting point:
2.4GHz - 12,988 + Upgrade 1,050 = 14,038
2.53GHz - 14,088 + Upgrade 350 = 14,438
Difference - HK$400
Buying a 3rd party drive or external drive is possibly slightly cheaper. The poster is at school and is likely to take good care of the machine (otherwise there is applecare or insurance). He does seem keen to maximize the speed and storage of his machine so let him.
I still use my MacBook Black from early '08 since then Snow Leopard came out (I upgraded), the computer works fine, with 2.4GHz, 250GB HDD and 4GB RAM. I use it for both work and pleasure (v large macro-laden spreadsheets, photo editing, plus general work).
I have also used macs since the LC475 and have found with them that getting more RAM and space if you can afford it, allows your machine to be useful for longer before support and upgrades cease. (I have converted my old iBook G4 to YDL to keep it speedy).