Top then shift m, which will sort the processes by mem usage. From the screenshot, there are about 170mb file system buffer and block device cache. Those are normal. You actually has more free ram than u thought
https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/5/html/Tuning_and_Optimizing_Red_Hat_Enterprise_Linux_for_Oracle_9i_and_10g_Databases/chap-Oracle_9i_and_10g_Tuning_Guide-Memory_Usage_and_Page_Cache.html
However, the os started to swap. U will have to figure why that happened since u still have free memory available