You can use a program like acronis or partition magic to do this.
You would first have to change the partition size on your H: drive to say 40gb (leaving you with 40gb more). Then in windows provided both disks are "Dynamic" you can use the "Spanned" feature which basically takes the 40gb of unformatted space on your h: and makes windows think its part of the c: giving you ~70gb on your c drive. Only problem with this is if one drive fails you loose whatever section of data is on that drive and youll probably need to reload windows because it wont like loosing half the partiton.
As for the smaller partitions on a single disk, people do this for a number of reasons. The first partition (usually OS) is generally the faster portion of the disk, second partition next fastest and so on. So intheory it yeilds a performance advantage by having more commonly accessed data on the faster sections of the disk. I dont do it for that reason though, I do it usually with 2 partitions this enables me to reinstall windows easier when i want to because the 2nd partition (larger than the OS one) can be used to backup data from the original OS install, while im formatting for re-install.