If you have a $700 budget, I would get as close as possible to that, especially if your mother keeps the PC for several years. If this is the case, buying the best you can get for your budget is always key since in a few years, it will be out of date.
Case:
NZXT BETA EVO Classic Series CS-NT-BETA-EVO - I got this since it is a simple mid tower with free shipping. $59 (Before $20 mail-in rebate)
Motherboard:
ASUS M4A785TD-V EVO AM3 AMD 785G HDMI ATX AMD Motherboard - A solid board that uses AM3 CPU's, DDR3 RAM, and Crossfire. I chose this because it has on-board graphics which means you do not need to purchase a dedicated GPU to run the PC. Also, I would stay away from micro-ATX motherboards for common desktop PC's, the components are more tightly packed and they tend to generate more heat. $91
CPU:
AMD Athlon II X4 620 Propus 2.6GHz - If there wasn't a tight budget, I would suggest one of the Phenom II's, but the Athlon II will work well for your application. $99
Power Supply:
Thermaltake TR2 W0070RUC 430W - Since you're not running a powerful GPU on this system, a simple power supply is all that is needed. This can easily handle the power requirements, but is also powerful enough to handle a GPU should you choose to install one. $36 (Before $12 mail-in rebate)
RAM:
CORSAIR XMS3 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600 - In a few years time, more than two gigs of memory will be common, so buying 4GB of RAM will be a good choice. More than enough RAM for most users. $92
Hard Drive:
Western Digital Caviar Green WD5000AADS 500GB 32MB Cache SATA - Plenty of space. $54
Monitor:
ASUS VW198T Black 19" - Asus makes some very good monitors. This one also comes with speakers built in, though, audiophiles won't use them. A PC designed for work doesn't need much larger than a 19 inch monitor, unless the workplace is some kind of graphic arts studio. $119
Optical Drive:
LG 22X DVD Burner - Bulk Black SATA Model GH22LS50 LightScribe Support - You need a basic drive. This one can read and write to most optical media. $28
Operating System:
Windows 7 Home Premium 64-bit - You need a 64-bit OS to use 4GB or more of RAM. $104
Grand Total: $691 (Before Rebates and Shipping)