if you want amd, don't go to dell (i don't think they make any comps with amds). otherwise, dell often has nice coupons and/or percentage discounts off of their computers. just check out some deals sites. here's one:
http://www.extremeoverclocking.com/deals/
but you can also go with other companies (emachines are nice and cheap). but, as stated above, it isn't a good idea to buy one then upgrade it yourself. the tradeoff with buying pre-built comps is that they usually have crappy video cards. if you think you will upgrade, make sure it has an agp slot (i think you can find those specs online).
as for specs:
2.8ghz - 3.2ghz p4 OR 2800+ to 3200+ Athlon XP OR some kind of AMD 64-bit proc
512mb ram (no lower)
at least 80gb harddrive (because i've never seen a hdd under 80gb with 8mb of cache)
cd-rw and dvd-burner
video card...just not onboard/shared...please! for $1200, you can do better than that...a 9600pro to 9800pro (ATi)
perhaps a soundblaster audigy 2 sound card and a nice set of 2.1 or 5.1 speakers
oh, 19" lcd would be nice. dunno much more on lcd monitors tho
good luck finding a nice comp. for $1200, you can definately find one