YES! Use the eMachine! Get some RAM: even 512 MB of it will be plenty for Ubuntu or Windows 2000.
If you can't, then get Puppy Linux. It should do basic stuff.
As for upgrades, you'll be held back by the CPU: try and find a compatible one if you need to, and maybe more RAM...
Third rig: PC Express Invader. C2D E6400 2.13GHz, 4GB DDR2-800, 320GB HDD, Intel 945, Asus 9600GT 512MB, 550W PSU, W7 HP 64-bit.
Fourth Rig: Dell Optiplex 170L, P4 Prescott 2.8GHz, 512MB DDR266RAM, 80GB HDD, FX5200PCI 128MB, 250W PSU,XP Pro
Fifth rig (Laptop): Dell Inspiron 4000, 14.1", PIII 850MHz, 384MB RAM, 9.3GB, XP Pro. Now with passive cooling!
“I could make a better looking computer case with a bucket of Lincoln Logs.” -lemode