first I will start by recommending that you do not buy from tigerdirect, as they are expensive. xoxide.com has that same case for 10 bux cheaper, and most likely cheaper shipping too. http://www.xoxide.com/aspire-x-plorer-case-black-silver.html
the only reason i would use tigerdirect is if shipping is expensive, but i live by one of their retail stores (yes there are tigerdirect stores) and buy it there.
as for the power supply you cannot go cheap on a power supply. It will cause many problems. 420 watts should be fine unless ur building an uber gaming rig, but that doesnt sound like what ur doing. there are only 2 name brands that i would buy a power supply from, OCZ (high end) and Antec (mid range). for you i would suggest going with an antec. they are good psu's and you get good bang for the buck. there is a good deal on a 450 watt OCZ that I would suggest getting. it is only $67 after a mail in rebate. http://www.newegg.com/Product/Product.asp?Item=N82E16817104154
here is an antec for $70 not quite as good as the OCZ but still a very good power supply http://www.newegg.com/Product/Product.asp?Item=N82E16817103936
Just make sure it has the right connections for ur proc, Intel or AMD. I would go with that OCZ.
when its mounted in ur case, the fan will be on the bottom. it is the intake fan, and blows out the back. it should fit in any standard atx case. again i would suggest either antec or ocz. 4 psu's ago i didn't think they mattered that much, now i know how important the psu is to teh comp, especialy vid card if ur a gamer. don't go to cheap.