Well, yeah. You could exchange the Phenom II X2 555 for an Athlon II X4 630 if you multitask, otherwise for games like WoW, which I believe is single threaded, thre Phenom II X2 555 will outperform it by quite a bit.
The overclocking option is just saying whether you are planning to overclock or leave the components at stock speed, iff you want to overclock later though once you learn about Phenom II overclocking, a great aftermarket cooler would be the Cooler Master Hyper 212 Plus, for $34.99, although you might need to by an AM2/AM2+/AM3 mounting bracket.
I thought you might like to know, CrossFireX and SLI are two different but similar technologies, where you run two or more different ATi GPUs (CFX) or nVidia GPUs (SLI) to boost graphical performance in games. The reason why I say GPUs instead of graphic cards is because some cards are CFX or SLI enabled by default, these cards are dual GPU cards which are for example the ATi Radeon HD 4870X2 and nVidia's 9800GX2.
Well, if you're going to crossfire you can go wrong with an ASUS M4A79XTD EVO motherboard. The Antec 300 Illusion for around $60 last time I checked will offer more than enough space you need and keep your system cool. You'll also need a 600W PSU, you should stick with brands like Corsair and Seasonic. A few antec PSUs are nice though, and maybe you could also try out OCZ.
You can also fit an AMD Phenom II X4 955 BE CPU in there if you find a few combo deals and for more price cutting you could go with a cheaper 5770 from ASUS, MSI or Sapphire. No point in the Vapor-X edition unless you're overclocking. The rest Lmeow mentioned is decent, not sure if you can find a nice pair of memory sticks from G.Skill around the same price point.