I agree with LowJK. You more RAM you have the better, especially if you´re going to use multiple softsynth plug-ins. You will find yourself running out of RAM very quickly if you record a few tracks AND running plug-ins at the same time. Windows XP itself eats up a lot of RAM. 1GB and above will be safe.
Why Cubase and Logic on the same machine? You only need one since they are duplicating each other. My DAW has only the bare essentials - keeps everything running as stable and as fast as possible.
As for MIDI interface, Steinberg´s interfaces (MIDEX may have an advantage if you´re using Cubase due to LTB, as suggested by LowJK. MOTU is pretty good too. If you´re going for Logic, then emagic´s AMT8 may have the advantage of tighter MIDI timing. I have a dislike for USB MIDI interfaces basically because of poor latency issues. This may not be a problem now as it was in the past particularly if your CPU is fast, but I still stay clear of USB just in case. This is only my personal preference (in fact, I disable all my USB ports as well as all unused PCI slots in the bios - you´ll be surprised how much CPU resources that frees up!).
Hard drives - SCSI is the best if you have the money. Go for U160 or U320 SCSI controllers. If you´re on budget, a fast 7200rpm IDE should be enough. Just make sure that the HD for recording is dedicated for just that use; Windows and all other programs should be installed on one HD other than the one for recording. Looks like you´ve already got your HD. If not, check the website for the specs of the HD you want to get. Different drives with the same rpm perform differently. What you really want to know is the write and seek time - the lower the better.
Soundcards - M-audio is good, as recommended. You really want to make sure the card has the IOs you need. I´ve no experience with M-audio, but I´ve been using Echo for many years with good results - very low noise, very low latency, multi-client (can use more than one audio application at the same time). Echo Mia may be worth a look - 4 analog IOs (8 virtual audio outputs), S/PDIF IO, 24/96 , relatively cheap. Swizer in Singapore sells it; otherwise check out Ebay. I get some of my stuffs there really cheap.
One issue you want to ponder upon, particularly if you´re going to record vocals, is the problem of noise. Some HD are noisier than others. 2 hard-drives, CPU and casing fans etc creates quite an amount of noise and can be a problem if you´re recording vocals. My machine sounds like an aeroplane every time I turn it on (CPU fan, casing fan, 1 IDE drive, 2 SCSI drives, 2 drive cooling fans). I´m open to suggestions...
Oh, by the way, what is your budget?
<font size=-1>[ Edited by Cheez On Date _MONTHDATETIME ]</font>