Best way to improve my wifi?

On a budget of about 100 quid max, what's the best way to improve my wi-fi range. Moving the router, a Netopia 2247-02 I think (the bog standard one that came with Eircom broadband) isn't an option so i need to boost it's range.

I only really need to extend it by like 20-30 feet to get the whole house in range. The signal now reaches about 40-50 feet (that's a bit of a guess, whatever the average length of a small-medium house is) and goes through 2 internal walls but would have to go through an external, concrete wall (small extension added to house) to cover the whole area.

Having done a bit of research, the options open to me seem to be:
1. Upgrading my router,
2. Using an old router as a repeater,
3. Buying a dedicated repeater/extender, or
4. Powerline kits.

These all seem to have their positives and negatives and I was just wondering what people thought might be my best bet?

Cheers in advance.