"readyboost" is a flawed theory. I guess as a last resort it has its merits though.
You would be much better off using part of your hard drive for swap if you can. It will be substantially quicker. The reason ram is preferred over swap is because hard drives are so much slower. A usb memory stick will be even slower than a hard drive.
In addition to the detrimental aspect of speed, using a usb memory stick as swap can be very harsh on the memory stick. Usb memory sticks implement a type of storage that has a limited write life, meaning you can only write to the device XYZ amount of times before it is "dead".
The first thing I would recommend to you, is find out what files on your hard drive you can safely delete to make room for on-drive swap space. If you need to, it would be better to re-locate some of your files to the usb memory stick so that you can use on-drive space for swap.
Secondly, and I realize this might not be an option depending on your situation. Look into budget-friendly upgrades for that laptop. On many older laptops you can do some searching and find people almost giving away ram. Upgrading the hard drive could be a little bit more difficult depending on how old the laptop is. Some older laptops used propitiatory drive forms or carriers that made it difficult to upgrade. That is pretty much standardized recently though.
Third suggestion, is to be mindful of what applications you are using. This is probably the most difficult of the 3 suggestions. In Vector, we get several different choices for most "type" of application. Finding the right application for each "type" for your situation can be time consuming and can take a considerable amount of learning curve. For example you could choose for a file manager Dolphin, Thunar, PCManFM, Rox, MC, or a bash prompt. The file managers I have listed are in approximate order of using the most ram (Dolphin) to using the least ram (bash prompt). You will find similar situations with web browsers, desktop productivity software, and even media players.
I hope this helps give you some ideas Carsten!
P.S. I too come from the PHP/web dev world. I got heavily into linux while trying to optimize my web servers to run PHP apps more smoothly. I don't do much in PHP anymore, but the few PHP things I have or develop on run on a server that has 52MB of userspace ram. Playing in resource-minimization and optimization is a great way to learn more about linux!