things to look for:
-dual-core processor, preferably intel. huge difference in quality, reliability, and performance. don't go lower than a pentium dual-core (don't touch anything with celeron in the name), and remember that FSB (where the cpu connects with the system) is just as important as clockrate. my wife's is a 2.2ghz pentium dc, and it's pretty nice. handles vista pretty well (nothing compared to a desktop, though).
-whatever you get, max out the ram as soon as you get it. laptop ram is stupid cheap - like 4 gigs for under a hundred - and really easy to install yourself. plan on it when you buy. ram generally costs a lot more from the salesperson than from newegg (and they rarely stick one larger chip in, they'll usually clog your ports with two smaller ones) so buy low and install high.
-large, bright screen. seems dumb, but screens die with age, so aim for one that's more than enough for what you want.
-not much bloatware. all that extra crap just slows you down.
-discrete graphics are pretty expensive nowadays for some reason, probably related to the high cost of PCBs. buy accordingly. if it's nvidia, don't buy anything that's got numbers lower than 8500 in the name. 9600 would be excellent, but it's pricey. the 9500 is great, an 8600 is definitely capable. the 8400s and 9400s aren't worth the cost of the solder on the chip.
-hard drives are cheap. if you 'order up' on a custom-built deal, make sure it's worth the cost. check newegg or something. if it gives you a choice, choose a 7200rpm hd over a 5400rpm, and choose standard over ssd. sata over ide/pata of any kind. if they offer two drives, don't buy the second from them. just get one and buy the second when it goes on sale for half the cost, and install it yourself. there's not even any plugs to worry about with lappy drives =)
whatever she gets, wipe all the crap off of it immediately and load firefox and all that on there right away. that other stuff is terrible for performance.