Though your GPU will run with your current power supply,I would suggest upgrading to at least 500W to cover any future upgrades you may make. As I am not sure what you mean by reasonable price I have tried to suggest ones that I might purchase if I was working to a strict budget.
Yes a couple are 450W but they will still be a decent upgrade.
There is nothing particularly difficult changing a power supply, just make sure that as you remove the old PSU's cables you take a note of what they are and where they go. When I first started out I photo'ed each cable as I removed it from the motherboard and also where it went. Just take your time and you will be fine.