as for Search Engine Optimization, what is the best way to write numbers?

Question

with letters (eg. two, forty-five) or the number itself (eg. 2, 45)???

Answer 1

both are read by search engines as a string of text characters - they can't tell that '2' and 'two' are the same thing (not yet). So it's all about what people will be searching for.

A lot of times this comes down to common sense, for example in the example given above for PS2 - people just won't type 'PS two' in the vast majority of cases.

Proof - http://www.google.com/trends?q=PS2%2C+PS+two%2C+ps+2

That's Google Trends. You can use it to compare the approximate search volume of two or more terms to see which one you should use throughout your website. In the above example I've compared PS2, PS two and PS 2 - you can see that PS2 is the only one people use.

Give it a go!

Answer 2

The best way is to write what the user will use when typing a search term into a search engine.

For example will they be searching for a book 'tale of two cities' or a game for the 'PS 2'.

Use whatever the searchers use.