you do want to go 64bit, forget x32. As for the rest I agree with Steve it's just difficult to know. I will disagree about NIC's there should never be one of anything - single point of failure, you'd want at least two nic's probably teamed.

I've just spec'd a DW server - x64 4 x quad core ( although I'd prefer dual core AMD ) 64gb ram 42 15k disks + 10 internal sas. Note that most SAS disks only have a 10k spin speed so 15k scsi's will give you slightly better throughput and response.

I only have an estimated 100Gb of data at this time.

However it's really difficult to work it out - we were doing a data load test this afternoon - seems there's maybe a bit of a scaling issue - after 3 hours we'd managed about 10% load , run a 4 way quad core at 90% cpu solid and done the most io I've ever clocked on a query, 4.5 billion logical reads.