Pro Football Talk offers updated cap numbers as of March 30th (more details from La Canfora here). The Seahawks are listed third, with $18.9 million in cap space. This fits in well with our own Davis Hsu's estimation, which is about $20 million. None of the other NFC West teams have anywhere close to that much space left: 49ers at $5.3 million, Rams at $4.3 million and Cardinals at $2.1 million.
The top-51 rule is currently in place, so all those numbers indicate the top 51 paid players on your roster. Future contracts, UDFAs and low draft picks, as well as some of the lower minimum salary guys don't count against the cap at all, if they don't break the top 51. The Seahawks have about 65 players on the roster, and the roster max for the off-season is again 80. The entire rookie pool is signed for $5-6 million, but that's the money they make, not all of them will be in the top-51 and they will eventually be cut or displace more expensive players, so you don't literally need $5-6 million "in cap space" to sign your rookies. K Steve Hauschka and DT Clinton McDonald are restricted and exclusive-rights FAs who have not yet been signed.
We've got a ton of space, but there's not a lot left to spend on. Other than the usual roster churn Pete Carroll runs every year, I'd like to see a starting-level LB added, but the LB market has been putrid so that really shouldn't cost much of anything. David Hawthorne is the only free agent of any stature left that I could see us being interested in, and I doubt he'll cost more than $2-3 million in his first year.
Do we *need* to spend? Not really. Davis discussed this a bit on twitter before. I went back to check the CBA. The cap rollover is discussed on page 96, and it makes no mention of any mechanism to stop a team from rolling over cap space continually:
Carrying Over Room A Club may "carry over" Room from one League Year to the following League Year by submitting notice in writing signed by the owner to the NFL no later than fourteen (14) days prior to the start o f the next League Year indicating the maximum amount of Room that the Club wishes to carry over. The NFL shall promptly provide a copy of any such notice to the NFLPA. The amount of Room carried over will be adjusted downward based on the final Room available after the year-end reconciliation
Year-end reconciliation deals with bonuses and the like. Since no limit on roll-over is mentioned, as far as I can see the Seahawks can roll over their full cap space at the end of the year, though I won't try to predict how much that'll be after further signings, rookie wages and bonuses paid out. So rather than "we have $18M we have to spend", the situation is more "we have $18M, we should try to roll over as much as we can because we have a lot of talented young players we'll soon want to extend". Following this next season, we can renegotiate/extend Earl Thomas, Kam Chancellor, Russell Okung and Doug Baldwin (as he was an UDFA, which gives him a two-season extend/renegotiate limit rather than the three-season one of draft picks), and Max Unger will be a free agent if he's not extended before. These are all players I'd like to see stick around.