Backup

Hi, I?ve a server with 118 Gb of data on a RAID5 array with 3 SATA 160GB. The array is divided in two drives, one with the data and the other with incremental backups. Monthly I also do an external backup to an USB Disk.
My problem is that with this amount of data, sometimes the backup fails, either for space either for the time needed or other error.
The software is Windows back-up.

Can you help me on solving this issue?
I don?t know if I should buy more storage, wath kind, other software?

Thanks in advance.

Paulo