if it's better then your current PC then go for it (as OhSnapWorld stated)
if not: check if any of the parts in that DELL can be useful for your current computer and throw the rest of it out as garbage.
You can track down the parts for it. you just need the specific model name. there are more Dimension 8300's out there.
they have different specs for different prices. (or did anyway)
i found this: Documentation
it's the complete list of specs on all the Dimension 8300 computers. now you just need to find out which one it is.
btw: if it starts up without problems i don't see why you would need to change the battery.