Increasing ram will not help much cause you already have 4Gb . Since you cannot upgrade cpu and gpu ,i would suggest you replacing your existing 298Gb HD to 650GB HD.
A computer will start to suffer performance problems when disk space is low, or when the disk is fragmented. Defragging the hard drive and adding disk space, either through partitioning or moving applications and data off the primary disk will improve performance in those cases. If you have plenty of space on your primary drive, just keeping it defragged will produce the best performance.
You won't find that more hard disk space will increase speed, but it may boost performance. When drives get full, data gets fragmented. With smaller size disks, and smaller allocation units, a program may need to visit many different areas of the disk to retrieve the information requested. In the case of smaller disks, this is harder to avoid. That is why newer larger disks don't get nearly as fragmented nearly as quickly as in the days of smaller disk sizes.