An alternative approach would be to go with a custom builder such as AVA Direct (www.avadirect.com). The advantage of this is that you know exactly what components are going into the computer.
In contrast, Dell frequently contracts with manufacturers to provide alternative versions of their parts exclusively to Dell, which makes it hard to know exactly what you are getting with a Dell computer. For example, the Dell Dimension 8200 uses a non-standard power connector on the motherboard, so if you want to replace either the motherboard or the power supply, your options are to (1) replace both at once, (2) buy a replacement from Dell (no idea how much that would cost, but probably more than Newegg), or (3) build your own power connector. An earlier PC I bought from Dell used an Intel motherboard which included bios code to operate an inexpensive SCSI controller card. (Part of the reason that this controller card was inexpensive was that it didn't include its own bios.) This SCSI controller wouldn't work with the Dell machine because Dell replaced the Intel bios with its own version that didn't include code to support the SCSI controller. If you are used to building your own systems, you should consider whether issues like these could be a problem for you.
Dell generally provides good value for the money, so I don't mean to discourage you from buying from Dell. I bought my most recent system from AVA Direct rather than buying another Dell because I concluded it was worth spending a few dollars more to get standard components and to know exactly what I was getting. I would suggest a Dell if the idea is that your mother will just use the system as is until she decides to replace it. Dell used to have better prices than Compaq, but that may have changed, so I'd follow the previous poster's suggestion and take a look at Compaq as well.