Hardware questions

Hi guys,
We are in the process of buying a new dell server.
Here is what we need to be able to do:
- we need to be able to do queries on tables that has 10-20 millions
of records (around 40-60 bytes each row) in less than 5-7 seconds.
We also need the hardware to be able to handle up to 50 millions
records on a few tables (5 tables in the DB).

Here is what we are thinking:
- Dual Xeon 2.8 Ghz
- 4GB DDR2 400 Mhz Dual Ranked DIMMS (is dual ranked or single ranked
makes any differences in terms of performance?). Do you guys think 4GB
is reasonably enough?
- 73 GB 15k RPM Ultra 320 SCSI Hard Drive
- Dual on-board NICS (is this enough, or Gigabit network adapter will help?)

Any input or suggestions is greatly appreciated.
Thank you,

Jun