It depends how fast the processor is. A friend of mine upgraded his father in laws PC against my advice from win 98 to XP and it slowed to a crawl as I told him it would. It was so slow that the father in law ended up asking me to blank it and reinstall 98 which I did.
The thing is, is it worth spending more cash on more memory for it. Depending on what memory it takes it could end up being a bit pricey.
I would check out the price of memory for it from Crucial click here and then have a think about it.
Your father is used to win 98 and the speed he gets from that, I don't think he will accept the speed of the computer with XP installed with that amount of memory, whether he would like it with more memory installed is debatable.
See what the others say and perhaps print it out when you have a few replies and see if that convinces him.