Well there's no way that USB 2.0 can transfer enough data so it will lag and I don't think there's any reasonable solutions.
HDMI needs to be >1.3 to allow 2560x1600 (capped at 1920*1200 or something), whereas dual-dvi-connection can manage that resolution, which you have. You don't have HDMI 1.3 on your card. That's the reason.
You can't really brake your computer by attaching external non-broken accessories to it via ports (PS. this is not a challenge).
You need an extra graphics card or a newer graphics card which supports 3 monitors. If you want to just attach an extra monitor, you can probably buy very cheap, low power eating Radeon (I prefer same manufacturer, because two display drivers sounds like an extra hassle). If you're into gaming, buy a new one completely and disregard the old one. The one recommend to you does its job fine. Actually you can not get much cheaper single one since nobody offers the necessary technology and connections with them.
Dunno about your power supply, if it's a packet computer, adding the graphic card suggested to you might cause some problems. Your current graphics card is really energy efficient (Yay!) but it also means that whoever build this might have gone really cheap on power supply (Boo!).
Weevil, you can not do any gaming using multiple monitors which are attached to different monitors with non-crossfire configuration (eyefinity) or use both graphics card in a single monitor gaming. That is not a big problem here and I actually wouldn't recommend SLI/Crossfire setups anyways unless using 3 monitors+, but that's another subject.