Yes, it will run a 64-bit OS.  It will not, however, support hardware-assisted virtualization (VT).

The 1800 will take up to 12GB of RAM, 2GB max per DIMM ... registered ECC, PC2-3200 (DDR2 400).

Hard drives ... it depends on if your system is using SCSI or SATA drives.

SATA

• If you are using the onboard SATA ports (CERC SATA 2s), the max is believed to be 1TB (750GB for sure), max 2 drives.
• If you are using the SATA RAID card (CERC SATA 1.5/6CH), the max is 1TB, max 6 drives, 2TB max volume/array size.

SCSI

• Regardless of whether you use the onboard SCSI controller or an add-in SCSI or RAID controller, 300GB is the max size of drive.
• If you have a backplane (drives are accessible/removable from the front of the system), then you need 80-pin SCSI drives.
• If you have cabled drives, then you need 68-pin SCSI drives. 
• U320 drives are supported.