I got my vps from vpswebserver following plan for my forum

[Windows VPS Plan 512]

RAM: 512 MB
CPU cores: 2
Disk space: 50 GB
Data transfer: 500 GB
Operating System: Windows 2008 Enterprise (R2 also available)

Price first month: $12.45

their upgrade plan is

[Windows VPS Plan 1024]

RAM: 1024 MB
CPU cores: 2
Disk space: 50 GB
Data transfer: 1000 GB
Operating System: Windows 2008 Enterprise (R2 also available)

Price first month: $24.95

i want to ask when should i upgrade my vps plan as my forums users increasing and 2nd question is can i get cheaper plan then it ?????????