Xenesis: At the risk of repeating myself again, might I point out that you could get a significantly better HDD in terms of size/technology by just making a small investment like Rs. 500.
For RAM, I use the following key for myself to get similar performance numbers across both operating systems:
2 x (RAM requirement for XP) ~= RAM requirement for Vista
So on a similar config on 2 systems with just he RAM being the differentiating factor, in terms of performance,
1 GB RAM in XP ~= 2 GB RAM in Vista
Additionally,
1. The RAM prices are at an all time low. so now is a good time as any to go for as much as you can fit in the budget.
2. Vista makes much better use of more amount of RAM than XP. It actually caches all possible stuff in your available system RAM, making minimal use of your swap/page file; thereby speeding up your system significantly.