Really, almost any computer you would find at either Dell or HP would do for the types of things you mentioned that you are planning on using the computer for. For XP, I recommend 1GB of RAM over the more typical 512 MB and 2GB if it fits within your price range. Get a decent-sized disk in the range of 200-250 GB. If you go with Dell and one of their "home" computers, you have no choice but get Vista. If you get Vista, you should plan on having 2GB of RAM. You can order a Dell via their small business selection and get XP. I haven't heard of too many people having problems getting a PC with Vista on it from the manufacturer. (Those of us with home built PCs .. well, that's a bit of another story.)
You probably should get a reasonable pair of speakers for your music (or even a surround sound set if you are going to regularly watch DVDs). You may want to order no speakers from the manufacturer and visit a local Best Buy or Circuit City to get a set you like.
If you go with an LCD screen (likely), try to get one with a fairly small refresh rate (16 ms or less; 8 ms or less even better) and good contrast ratio. When watching DVDs on an LCD with poor contrast, dark areas tend to become muddled black on black rather than greys on black. A slow refresh rate tends to display "ghosting" as the display can't change fast enough. Get one at least 17" in size and 19" is even more the norm.
This assumes that you only want to browse the Internet, watch DVDs, play music and the like. If you or your daughters intend to play games on this new PC, you'd want a better graphics card than found on the typical sub-$750 or even sub-$1000 models in some cases. Such a video card could (almost always) be added/swapped in at a later time. A larger hard drive wouldn't hurt, but even a "small" 200 GB drive takes a while to fill (even with games running 6-7 GB each as they are lately). The better LCD screen is even more important in for games. Having 2GB of RAM for today's games is almost a must any more.