You need to take a look at your mainboard to see what sort of graphics card can be fitted.
PCI Express would be best, but you may not have this facility. Is there an AGP connector?
The memory you buy has to be suitable ie can the mainboard handle this much and is it the correct speed etc.
The graphics card you mention is not very fast for games by todays standards.
Forget upgrading the processor. This would almost certainly mean a new mainboard and virtually a new PC.
The 2 seperate drives C and D are probably partitions of the same drive.
It is better to partition a large drive in this way as it makes maintenance easier. There is no speed advantage in one large partition.
You don't mention the operating system, - is it XP? If so the FAT32 file system could be converted to NTFS which allows you to work with larger file sizes (essential for working with video editing etc.)