Given that it is almost impossible to find hard drives smaller than 80G, that would also be my minimum. For memory, at least 1 gig for XP, but 2 gig will give you better performance better. No need to go higher than 3 gig with XP. It can run with as much as 4 gig, but it can't use all of it.
With Vista, I would set my minimum hard drive at 120G and memory at 2 gig, but 4 gigs of memory would be better. Vista is a real memory hog.
Yes, both will run with much less memory, but with either XP or Vista, when the physical memory is used up, they start swapping stuff between physical memory and virtual memory (hard disk - which is very slow compared to physical memory) as need to support the applications that are running. The more physical memory your system has, up to its limits, the less it has to swap with virtual memory and the faster your computer runs.
With memory running only $29 per gig for DDR-2 and $45 per gi for DDR-3 memory, there is no reason to not max the system out with physical memory.
As for hard drives, they are so cheap, just get a big one. I was looking online today and I can get at 1 Terabyte (that's 1000 Gigabytes) drive for under $200. 80G and 120G drive go for about $50 to $75.
What I like to do is use about a 120G for my C: drive where I load only my operating system and programs. Then all my data goes on the D:, usually a 250G or bigger. That way, if the operating system screws up and I have reload it and the programs, my data is still in tact. Also note, I recommend two different physical drives for C and D, not just one large drive partitioned. With an arrangement with a partitioned drive, if you lose one partition, you most likely will loss all the partitions.