Interestingly, according to a recent survey by Quantum's Snap Appliance group, which makes entry-level and midrange NAS products, 35% of its customers responded that they were using their NAS devices as backup targets. According to Gartner 25% of NAS products are being utilized as backup targets. That said, follow a proven process.
The image below represents a low cost process for solving your backup problem. In this configuration, your Win2k and Solaris servers would be attached to a Network Attached Storage (NAS) device. Data on the servers primary storage is mirrored using simple host based mirroring to backup target storage volumes on the NAS device. Once data has been replicated to the NAS device the data may be backed up to the direct attached tape library (backup achieve) using the industry standard Network Data Management Protocol (NDMP).
The advantages of taking this approach is that once the data has been replicated onto the NAS device, you have in essence completed a backup. This design provides for centralized tape administration, rapid restore of data from disk, and the elimination of the need to purchase multiple copies of backup software. The disadvantage to this approach is that network traffic is increased when data is transferred from the server(s) to the NAS device.
Potential products or vendors to look at are:
Snap Appliance (Quantum)
To give you an idea of pricing for this type of solution Iomega offers a 1.3TB NAS backup bundle with a, 16-cartridge LTO-2 autoloader and CA ARCserve backup software for under $24,000.