Gross will cost a fortune being an all pro LT in 2008. Other guys out there are not very inspiring like a Vernon Carey or Khalif Barnes. Or they could go for an injury risk guy at a bargain like a Stacey Andrews or Marvel Smith. I'd also bet Runyan will leave Philly (they have a ton of FA's) and he could be a decent stopgap if they draft someone to groom behind him for a year or two.
OLB:
Suggs is the most natural fit as a FA, but will cost a ton. Baltimore may let him get away as Ray Lewis and Bart Scott are also FA's. Julius Peppers is also a FA and has said he will not return to Carolina and would prefer to go to a 3-4 team and play OLB. He may be available if Carolina decides to tag Gross instead. That would be a dream, but probably not very realistic. The pass rushers are thin this year, after the top two you're looking at an ancient Willie McGinest and some unproven guys.
S:
Unless they want to try Smith as FS, I don't think they have one on the roster. Goldson didn't improve in his second season as hoped, Roman is terrible and IMO Walt Harris can't play FS, he's more of a pure corner and still useful as at least a nickel, even if declining. For FA there are some big names like Mike Brown and Dawkins, but I can't see the Niners interested. OJ Atogwe is a FA and may be an option. No idea if the Rams let him get away, but he's pretty decent and may want to come here as a Stanford grad.
QB:
FA QB's stink. I think you have to draft someone. Too bad the draft is pretty thin at QB also.
I also would like to see them add a FB, maybe someone like Leonard Weaver from Seattle. They drafted a FB last year so they could let Weaver go. He's not a super blocker but pretty versatile and could even get short yardage carries.