[quote name='harrygetsoff']You're right, most of the time you will be ok and not experience any issues with wireless. My wii is online and is fine. But, the signal is susceptible to disruption from local devices and if you use encryption, you will get some performance degradation. From my experience, gaming consoles do not play well with WEP and/or WPA. If you are close to the access point and don't use encryption, you will probably be ok. You still run the risk of packet collision due to half duplex. Loss packets=lag[/quote] To the quoted poster, you're a complete idiot. Wireless isn't half-duplex. It's full-duplex.
WPA and WEP add overhead into the transmission of information, though the real bottleneck is your connection out to the Internet.
The main issue with any wireless network is how many devices you have, and if you're mixing 802.11B and 802.11G devices. If your network is 802.11G only, then you'll have an easier time of it than a mixed network. Also, having quite a bit of devices on your network talking to a single access point or router could cause slowdowns because the AP or the router isn't keeping up with everything going on.
To the OP, wireless can be added through an wireless bridge or wireless access point connected to the PS3's Ethernet port. The 60GB has an internal 802.11B/G card. If the 20GB doesn't have the internal connection for the wireless card, then it can't be added in the future. If it does, then in theory, a wireless card from a dead 60GB could be added to a 20GB. Voiding your warranty all the way, of course.
My Xbox and PS2 are connected to a wired switch (because my AP only has 1 Ethernet port) which is connected to a wireless access point running in bridge mode. Works perfectly OK for XBL.