Log in via ssh and run "free". It should give you an output similar to this:
Code:
root@asdf [~]# free
total used free shared buffers cached
Mem: 8187876 2270900 5916976 0 423864 1053936
-/+ buffers/cache: 793100 7394776
Swap: 4192956 0 4192956
As you can see it shows 8187876 KB or ~7.8 GB on the above machine. If you have 1GB you should see something along the lines of 1048576 give or take a little.
If you see something closer to 524288, then you need to talk to your VPS provider as the OS is only seeing 512 MB of RAM.