small office backup hardware advice

[Sdavtaker]

Hey,
Im working in a small office (4 PC with 80GB HD each and a "server" with 
a 200GB HD).
We were working with no backups at all until now, we was just 
replicating some critical data in more than 1 PC all the time, back 
then  there was no money for any hard copy of the data outside of the 
network (maybe a couple DVDR time to time).
Now, they gave me some money for backup stuff.
What will be a solid but cheap solution to keep some usefull backups?
If i do full copy i got to keep 200GB aprox of data per month, if i can 
backup diffs, then it will be like 100GB.
There is no big hurry on the backup time, since i can have a PC out of 
service for a whole day, maybe 2.5 in weekends.
Im a real newbie on the backup world :-(
Thanks for any suggestions.
Sdav