I really can't compare prices (I'm in NZ, the price to build that computer your posted would be almost $900). But I can still offer advice.
I think it's probably worth explaining nVidia's naming scheme so you kinda know what's going on :P :
- The Letters denote the "range" the graphics card is in. GS is worst, followed by GT, then GTS, with GTX being the best.
- The first number denotes the generation of the card. Lower numbers means the card is older (and therefore likely slower), a higher number means the card is newer and likely faster.
- The second and third numbers denote the actual processing power of the card (in a way), for example - A GTX 480 may be similar in processing power to a GTX570 (as the GTX570 is a newer generation, and the range of cards has shifted to a higher power, so the new GTX580 will be more powerful than the older-generation GTX480)
To then answer your question - YES, the GTX470 (high-end) is a much better video card than the GT520 (low-end). That said, after a little reading, it seems the video card may not be all that important for rendering... I would still spend a reasonable amount one though. Maybe get one of the older GTX260's?
As for CPU - I'd go for a Second-generation i5 (2500 maybe?)... (I'm similar to Giccin in that I like certain brands, AMD is ewww). I don't think the hyperthreading of the i7 would be worth the extra money you'd pay for it, at least not with current software.
Edit: you should be able to get a rig that's good for the job for well under $900, but you might have to go a little over $400.
