IMO, you can't achieve SAQ D (with local card data storage) PCI compliance on a Linode, and a firewall or anti-virus is the least of your problems. Disk is not completely under your control (at a minimum any Linode administrator can technically access it). Plus there's no way to completely isolate the physical network, which means every other machine touched by your Linode (or at least on the same fabric, but the network topology is not exposed) technically needs to comply as well. Particularly with SAQ D, I think you'll have a hard time answering a lot of questions that require implementation of items outside of your administrative control in a VPS environment.
You might take a peek at
viewtopic.php?t=5622 where this was also discussed - I think the net result was an agreement that you really don't ever want to touch card data if you have any way of helping it.
I used to think achieving PCI compliance (with stored data) would be unlikely in any VPS environment, but then Amazon went and apparently became certified as a service provider, so I suppose it isn't impossible.
-- David
Edit: Update to qualify overly broad comment about "PCI compliance" since some forms (e.g., SAQ A and probably C) should be ok in a VPS environment as they don't trigger as many of the physical controls as do card data storage and SAQ D. PCI compliance at the smaller merchant levels is also self-certification, so this is just my opinion.