Expanding Proliant DL580 G7 raid

Hi,

I administer a Proliant DL580 G7 on behalf of a research group.
Its raid setup currently consists of:

 2 x 150Gb 15K SAS in RAID1 for the OS (Scientific Linux 6.4)

 4 x 300Gb 15K SAS in RAID5 for the /home partition
 (the part number for these disks is 652625-002)

which leaves 2 disk slots unused. They now  want to expand the /home
partition so my question is - how near a match will the 2 new disks need
to be? The specific disks we are looking at are these:

http://h30094.www3.hp.com/product/sku/10385867/mfg_partno/627117-B21

I'm guessing I'm worrying unnecessarily but I'd just like to check
beforehand if possible.

Thanks for any input.