Does ReadyBoost really improve performance?

Yes, it works really well, but you have to get a 'fast enough' storage for this. It helps me a lot in my notebook. And it's good since I don't have to use a 7200 RPM HDD, nor waste money on SSD or something like this. I'm not made from money and a fast enough pendrive is dirt cheap now. Try it out with a friend's pendrive.

(I'm running Windows 7 Ultimate with 4 GB DDR3 1066 MHz low-holo Gepida memory (IBM-Lenovo ThinkPad T500), dual-channel. YET, it helps a lot.)

Yes, ReadyBoost will help.

But I think an 8 GB card will likely be overkill - with ReadyBoost, more is not always better. ReadyBoost works as an optimization for your existing RAM and page file. You're not storing or caching more information anywhere, you're putting the same information in a faster location.

Your page file rarely grows as large as 8 GB. It does happen, but most of the time it's sitting much closer to 4 GB, and so I recommend looking for a faster 4 GB card. That will be a closer match for your actual use. Check reputable reviewers for actual benchmark data, as the faster the card, the more the speed boost.

Even better, most netbooks support up to 2 GB of real memory. ReadyBoost will help, but the 2 GB upgrade will help more.

It can help but only if you have minimal RAM. Benchmark tests by various sources have been done to show its effect:

Toms Hardware Windows Vista's SuperFetch and ReadyBoost Analyzed

As you can see, if you have 512 MB RAM, ReadyBoost can really help. If you have 2 GB or more it probably will go un-noticed.

For you with 1 GB ReadyBoost would help slightly, a 4 GB or 8 GB USB stick would work best (see the other link in my comments), although for the same price you could probably get a 1 GB RAM upgrade which would be better.