All of those sites are great. Dell offers deals on Inspirons (the consumer line) a lot more than Latitudes (the business line). The big coupons usually show up around the end of the fiscal quarter. This quarter ends July 29th. The week before that, all of those sites like slickdeals will be full of coupon codes.
Just an aside: I visited the DELL site through my schools website, and i was surprised to see the discounts that were offered were pretty substantial. So if you can try the student discount thing too, that may help you save even more.