If you don't care about doubling up within the same alliance, and just want to ensure unlimited complementary upgrades domestically, getting the Barclay's USAirways-branded mastercard might be the way to go. Spend $25K and you get 10K EQMs, plus 5K discounts on awards, and some Silver Preferred-like benefits. Get three of the cards and spend $25K on each, and you get full SP status plus some to spare. (At one time, I did this with six cards to earn Gold).

If you haven't maximized your status tier on one of your existing airlines, adding more Delta-branded Amex cards might be the way to go. With two Delta Reserve cards (one consumer, one biz) and two Delta Platinum cards (one of each, also), you can spend $220K amongst the cards every *other* year and earn 100K MQMs/EQMs in alternating years... just 25K shy of the new top-tier Diamond Medallion status and enough to keep you there if you add the other 25K by flying, etc.

You mentioned only airlines, but if your interest in elite status extends to hotels, I think Amex has a Hilton-branded card that gives you Diamond status with them for $40K spend, the Starwood-branded one gives you Gold status with (I forget - maybe it's $50K annual spend?), and the Chase Marriott Visa Premier Signature card recently announced that $180K annual spend will get you Platinum status with Marriott. (The Marriott card can be done every other year and you'll keep the status uninterrupted... I'm not sure about the former two. Someone else should know.)