Definitely, with a $400 budget, your choice would either have to be the 3600 series iPAQs or the UR There @migo. Ok, the @migo is selling for $400 right now at a couple of places I know of. Like I mentioned previously, it has some advantages over the iPAQ like the integrated PCMCIA slot and the 16-bit color display. I forgot to mention the larger battery capacity of the @migo as well (1550mA vs 950mA for the naked iPAQ).
However, if you're really looking to save money, there are a few deals floating around for the iPAQ 3635 and
its $150 mail-in rebate. Most of them deal with pricematching an online retailer to Staples and using a coupon. It was just mentioned today that Radio Shack has dropped the price of the 3635 down to $350 so that would come out to roughly $200 after rebate. Not too shabby at all.
As long as you're aware that the PCMCIA sleeve adds significant bulk to the iPAQ (some refer to it as a "brick" with the PCMCIA sleeve on) and can live with a 12-bit color display, go for the iPAQ deals.