Samurai Edge:
You can't link between Fire Red/Leaf Green and Ruby/Sapphire unless you unlock that feature in the Fire Red/Leaf Green games. Also Ruby/Sapphire do not support the Wireless Adapter function.
XPWaRRioR:
Yes VBA Link does work perfectly for most GBA games as long you set it properly. The default link settings will work for most games though. I haven't used v1.8 yet but I'm getting it now. Since it is still in beta stage I'll try it out and if I find any problems with it that aren't just the same problems that the real VBA have I'll let you know.
Ok, I've worked with it for a little bit and there are so far two issues that I ran into:
1. The link feature uses a lot of processor power and can cause the emulator to slow down. When I ran it both vba link windows were running at about 50% and my processor usage stayed at around 99%.
2. There appears to be a major bug in the Wireless Adapter feature. When I tried to communicate with someone in the Union Room on Pokemon Emerald the emulator crashed and the connection was lost.
Other then that it works alright. I highly recommend this software if you want to be able to link on your computer (even with it's major bugs).
How to link:
1. Copy all the files from the downloaded zip file to a folder on you computer.
2. Open the VisualBoyAdvance.exe file. Now open it again leaving the first window alone.
3. Set each emu windows controls.
4. Open a GBA game in each emu window. Note: the games have to be able to link to each other in the first place for the link to work with the emulator.
5. Start a link game on both windows and enjoy.
If you still have questions let me know. I've been using VBA and VBA Link for several years now and have learned quite a bit about each. Also if you plan on playing any Dragon Ball Z or Pokemon games check this out first:
http://www.1emulation.com/forums/index.php?showtopic=17613