Is there a good way to rescale textures...

The only way is to repaint it

Poop in my mouth has an article about that http://www.poopinmymouth.com/tutorial/resize.htm

Well that was more of scaling down really.

But yeah you really can't scale up something to the double size with a good result without editing it quite some.

Also I've never been very fond of doing stuff in double res and then scale down as poopinmymouth says in the article I totally agree with that.

Of course in some more rare cases it might be good (if you made a pack for some newer game i.e. hl2 and want to port it to hl1 as well obviously )

You can make it work by playing around quite alot with many of photoshop filters. I did this kind of experiment a few years ago and the result turned out pretty nice after a couple of hours of experimenting. Too bad I didn't have knowledge in ps actions back then

To sharpen it up you can try to use a high pass filter. What I often do to sharpen an image is, instead of using the real sharpen filter, duplicate the image layer, desaturate the new layer and then run Filter>Others>High pass with a not so high number on it > then set the layer to vivid light.

try scaling the file up 20% five times, so it's the size you want

photoshop should do a good job doing so ...

edit:; oh wait... if you scale it once the size would be bigger so 5 times would be to much .. right ? or am I thinking too hard ?

yeah i think that would involve some math.. and not sure if it'd be worth it...

i kinda wanna try your method of sharpening hourences.. sounds intriguing.

There's no real quick fix when doing something like that because you're trying to create high detail from low detail. It's a bit like taking a 96kpbs mp3 file and recoding it to be 320kpbs -- it won't sound any better just because it's got more data in the file because the data is just the old stuff but padded out to fill in the blanks.