If you like, switch to Windows 7, which will boost your performance quite a lot. I have two T60s, and running Win7 is like bleeze. XP is too old, bundled with Lenovo SW gets even worse. Yes, the optical-bay box for HDD is a must have. 30RMB in taobao.
For the heavy computation side ... maybe getting a desktop with 6 core or 4 core is a better solution.