If this was my build, I'd switch on the graphics card and memory for sure as those are just $5 each more for EVGA and G.SKILL, and I would probably switch out the power supply. I might switch on the motherboard, that is a pretty good deal though and $10-20 might not be worth it.
I decided to change the hard drive to the one you suggested, but his budget is really tight so I changed the GTX 460 to a HD 5770 and kept the same motherboard, ram, and psu. He needs to be able to transport his computer sometimes so I changed the case to the coolermaster storm scout. The grand total came out to $426.
Starcraft II is generally a processor-constrained game because of the many units--and is relatively light on graphics because framerates aren't that important for RTS.
Your graphics shift wont be a problem. My ATI 5770 is not challenged by SCII in the slightest -- however my E8400 occasionally stutters when many many units stack up in a game. Ideally he will be able to swap out the RANA for something better in a year or two.
In conclusion: I like what you came up with -- but if you suddenly decide you have more money spend it on the processor-- not the graphics.