Both the standard and Santa Rosa models have only two memory slots. Both of these systems have limits on how much memory you can add the standard models supports upto 4 GB of RAM (two 2GB modules) The Santa Rosa is able to support 6 GB of RAM (one 4GB and one 2GB module).
- Your system is the Standard one which will only support 4 GB of RAM.
You do need to make sure your systems firmware is upto date. Follow this Apple TN: EFI and SMC firmware updates for Intel-based Macs.
As to the HD you can use any sized drive but the drives SATA speed must match the systems SATA I/O port speed. Today its hard to find new SATA I (1.5 Gb/s) standard HD drives which is what your system needs (either model). Most HD's available to day are SATA II (3.0 Gb/s) or SATA III (6.0 Gb/s).
But you do have an option here! That also can give your system a bit more zip than a straight HD thats a SSHD (hybrid HD). Seagate makes a nice one and it has auto SATA speed detection. Here's a write up that describes what a SSHD is: Seagate Laptop SSHD. And heres the spec sheet for the drive: Seagate laptop SSHD spec sheet. Note the line item SATA Transfer Rates Supported (Gb/s) where all three SATA speeds are listed 6.0/3.0/1.5. Make sure what ever drive you get can work in your system like this one.