Do you have a standard case (that mounts a standard ATX power supply)?
If so, you're upgradeable - but you may not have to get a new PSU.
Assuming (since your CPU is recent) that you have a normal PCI-E vidcard slot, you could run an nVidia GT240 or an ATI HD5670 without PSU problems.
But you can buy a beautiful PSU, something name-brand like OCZ StealthXStream 450W with Active PFC, for only $50 bucks and get a much better quality of power than no-name low-watt PSU.
If you want hi-resolution modern games you'll need GTX460 1GB and a new PSU, $300 bucks.
But a 240 or 5670 are only $100 bucks and will plug right in - it's your call.