Note: This setup was intended to max out the $800 budget.
With that budget, you can get into S939/PCI-E territory easily, unless you want to get some remaining money off your parents.
A64 3200+ ('Chester)
G.Skill Dual Channel DDR PC3200 RAM
With this, you'll probably get to use the PCI-E 6600 in a future rig for your parents.
You get Dual Channel capabilities that's essential to S939. That leaves a little over $310 for the Case, PSU, DVD/CD opticals, and hard drive.
Seagate 120GB HDD
Total: $636.82 (w/o Case&PSU)
Since you have an $800 maximum, you can spend up to $163 on the case and PSU and maybe even bump up one of the components. Okay, so I under achieved, but that's a good thing in this situation, no?
Your folks would be amazed at how fast this thing boots up.