In my case it means too many interruptions during games with...

"I need to check my email"
"I want to play one of my games now"

I haven't tryed to install any older games lately, but I haven't had any problems with this game on my machines.

Windows XP, 1Ghz Athlon, 512MB SDRAM, 64MB Radeon 7500
Windows XP, 1.4Ghz P4, 512MB SDRAM, 64MB Radeon 7500

However I have kept myself an old computer specifically for older games. That way if I feel the need to play something ancient I can just plug her in and play without the need for DOSBOX or some other utility to slow the game down and stuff.

What does your wife use the computer for?
Internet? Email? If games then what kind? (big games or small shareware type)...

If its the case with most wives... Than I'm assuming internet, email, some word processing, solitaire or a gamehouse style game, etc... If that sums up your wifes usage then you don't need to go all super duper computer.

For usages like that I would recommend the low-end machines currently for sale. The machines that cost a $1000 and such are more tailored to gamers or people who do graphic/video editing.