Well.....
1) You said that you prefer the Dell S2340L, then go get the Dell monitor. It only costs $10 more.
2)Go take a walk around SLS and have a look at the various cases on sale. Casing is subject to personal preferences and there is no casing thats suits everyone. Do note that the casing should have at least a GPU clearance of at least 305mm as your R9 290 Tri-X is quite long. There is no point in getting a case if the card can't fit into the case.
3) Go to Xtreme Solutions, Tecdrome, Cybermind, Challenger, or Cyberactive to test out the mouse and keyboard. Peripherials are subject to personal taste and no peripherial is suitable for everyone.
4) Go to Cybermind or Challenger to test out the speakers. Speakers are also subject to personal taste.
5) Your system draws out a maximum of 485W at peak system load and hence I think a 550W PSU is good enough, especially when considering that the Seasonic S12G is a 80+ Gold rated PSU and not some cheap, liar-labelled PSU(If Seasonic is famous for that, I would have asked you to get the 1000W version if alternatives are not avaliable.). If you think the wattage headroom is not enough, get these PSUs:
-C.M SEMI MODULAR V650S (139)
-FSP AURUM S 600W (109)
-SEASONIC S12G-Non MODULAR 650W (129)
-SEASONIC G SERIES 650W MODULAR (145)
-SEASONIC X SERIES 650W (175)
-SILVERSTONE 650W 80+ GOLD (155)
-SUPERFLOWER HX 600M GOLD MODULAR (109)
-SUPERFLOWER LEADEX 650w GOLD (169)
-Thermaltake GRAND 650W GOLD MODULAR (159)
6) Get some case fans for extra cooling. The Corsair AF120s (28) and Noctuas are highly recommended here, but they are relatively expensive. Cooler Master's Blademasters(14) are a cheaper option though people say that that they are noisy. You can also ask t258jgn for some Yate Loons. He's got some for $8 each.
What you do to your rig is entirely up to you. It's your money, after all.
