Sorry I did not make my reply clear. The OP asked if he should by one or two new drives in addition to his existing HDD. I assumed that the comment about two HDD would be understood as still buy one HDD to go with the existing... anyway I'm digressing again (bad habit)
Your layout looks good, but I would make these changes:
Drive 1 : 320GB
Partition 1: 50-100GB depends on the amount of games and software you plan to install
Partition 2: 500MB - Temp Internet Files (depends on your surfing habits)
Partition 3: Rest of drive
Downloaded programs: Original downloads preserved to assist in future reinstallations -> should this be on a new partition?
No, just create a folder for them
Drive: 2 200GB
Partition 1: 5-10 GB - Pagefile (This is abit much but you never kknow when you will need it)
Partition 2: rest of drive
One thing to remember everyone has different opinions on this, read what everyone has to say and get a feel for what you think is best for you. What I put above is what I would do if I had those two HDD's.
I hope this helps