Check out ramseeker.com for buying memory.
My 2 cents on your dilemma would be to go to the Apple Store with a list of what your wife wants to use her computer for and what she needs to be able to do. Talk to one of the staff there and make sure all your concerns are addressed.
Macs play nicely with PC's now and even though I have Virtual PC on my Mac, I rarely open it. If you do get VPC, make sure you have virus software for it. I know of a person who got a virus on theirs, but the Mac OS was fine.
Be careful to realize you might be having to deal with a very happy wife if you get a Mac for her. She will fall in love with it.
Cheers! CC :)
P.S. Make sure you get an AirPort network going - it rocks!
There is a distinct advantage to using VirtualPC in avoiding viruses... and other problems. Set it up to save your documents to the Mac HD instead of the PC Virtual HD. Then simply make a complete copy of the Virtual C: HD file when it is fresh and un-infested. Use the original C: drive file for your VirtualPC operations. DO NOT USE IT TO GET EMAIL OR FOR SURFING THE NET!!!
If it gets infested, delete it, make another copy from your back-up and off you go. Takes about two minutes.