Hi,
First of all, before I put a lot of money in an old case with an older motherboard, I would load the game and see if you can get it to work without changing your power supply.
What you have to decide is if you want just a stopgap measure which is what a 250 watt power supply is. The computers of today that are not too powerful require a bare minimum of 350 watts if a major brand like Antec and Enermax. Most modern units require close to 500 watts. While a power supply of 250 might run this game, you will keep having the same problem as you add things. I think you should look for a bigger power supply so you can expand in the future without having to do this thing twice.
If you just want to skim by with the bare basic, then choose a basic 250 that is not too expensive, because they will probably do the job for this game, but will not permit you to upgrade to maybe other games in the future that require more power.
There is also the issue of AT power supplies and ATX power supplies, so you need to know what you have (I don't know your machine) before you can replace the one you have.
If you buy an enermax or antec of 450 or so, that would be my recommendation. However, that will cost more bucks than taking a chance that the 250 will work. Also, you must know that some power supplies have different connectors than some, so that also is a problem. It may mean more work even if you just buy a smaller power supply. There are also newer supplies with the 24 pin connector instead of the old 20 pin connector, so that also can be an issue you have to deal with.
If you look carefully, you can find a wonderful new case called Antec Sonata II with a 450 watt supply that will carry anything you want to. They are around 120 bucks delivered and you will have close to 100 for a GOOD power supply. Whatever you do, don't buy a no-name supply.
Hope that helps.