I just finished building a rig for a friend of mine who's requirements were similar to your wife's (he needed it for Photoshop and AutoCad).
He's the kind of people who upgrades his rig every 5+ years though, so he gave me a fair amount of budget. Enough for him to get him a core i5 2400 based system.
A system like that can get north of 600$ and you don't seem to be interested in investing so much or your wife doesn't need such a powerful system at the moment. Assuming your wife doesn't do some really heavy stuff (i.e. manipulating a 20gb photo or 3d model or something like that) - I used to own a phenom II x3 710 and overclocked it to 3.4ghz, it's a good cpu even at default clocks though. In my opinion you should also get an nvidia card such as the gt 430 since they're usually more optimized for photoshop than their radeons counterparts.
However, you should really ask your wife and see how her current rig performs to get an idea of what she needs. As to whether or not she needs a specialized gpu such as nvidia quadro, show her
this vid.
Getting a cheap system is fine, but if it's not gonna be enough for your wife you'll have to pay big time since this system is not upgrade-friendly at all.