That is a really old MB that only supports old cpus and original ddr memory. I wouldn't spend money on upgrading it. Also, your estimate for putting together a new pc is too low imo. Look at more of the 500-700 range.
Here is a build for a completely new computer (minus Windows, monitor, mouse+keyboard, etc).
If you want to reuse some old parts from your current build (i.e. case, power supply, dvd drive, hard drive), you would only have to get the APU, RAM, and a motherboard. From my list above, that upgrade path would total $255 before shipping and taxes (if applicable). These components would handle everything your wife would want to do superbly.
Considering that I want this system to last as long as possible can someone help point me in the right direction to what board and processor for $500 will last another 6 years (Hopefully). I've always been partial to AMD but considering that I don't game anymore I open to trying something new.