While I still far prefer lighttpd to apache (I find lighttpd far easier to configure), I'd agree that most of the RAM savings you'd get from moving from Apache to lighttpd/nginx can be had with Apache by using mpm_worker and fastcgi.
Your site traffic is pretty low, so I'd say that your resource usage is going to be more defined by how much RAM it takes to get all the software you need running rather than by how much extra RAM you need to handle load (as I said, your load is very low).
Your page doesn't sound media intensive. You're looking at 180KB for those extra images, which is really pretty low, and if most of these are on a limited set of files, the disk cache will help with that. Also, linodes have pretty fast disks.
For gzip compression, lighttpd at least will cache gzipped content (it doesn't currently compress dynamic content, relying on php itself to do that), although I couldn't say if Apache did. Back when I was running a site with decent traffic (~9 million pageviews per month), the bandwidth savings from mod_deflate more than justified the minor bump in CPU usage, and that was on a box with far less CPU power than a linode (Linodes tend to have an overabundance of CPU power).
Stuff like JQuery is all client-side, so that doesn't affect your resource usage (asynchronous stuff is a different story).
Linodes scale almost laughably easily; resizing a linode takes very little time. I've resized two 768s into 512s, and the entire process took under half an hour. That can also be reduced to a single reboot if you're desperate to limit downtime by buying extra RAM on the current plan level. Economically, buying extra RAM doesn't make sense compared to upgrading the linode, but it does mean that you get 60 second of downtime instead of half an hour.
Because of this, I'd suggest starting with a 512. Start there, make sure you tune everything properly, and see what your memory usage is like. If you don't have enough breathing room, it's stupidly easy to upgrade your linode.
10-20% growth is awesome (good for you!), but easy to handle. If you don't mind setting aside some scheduled maintenance every few months to upgrade your linode, you should be golden.
Assuming you're going to keep growing at 10-20% for a long time to come, you should plan ahead and prepare for horizontal scaling rather than just vertical. While it's easy to upgrade your linode, disk IO does not necessarily scale linearly; at some point it might be more useful to split the load among two or more linodes.
The first step in this can be as simple as separating the database server from the application server. This is pretty trivial, as you just move the database to another linode and update the host/ip that your software connects to. From there, things can get a bit trickier, but if you're going to grow indefinitely, eventually your load will become extremely large and you should think about these things ahead.
This isn't a linode concern by any means, the question about how far to scale vertically before you scale horizontally applies to everything, even colocated servers, although in that case it's often because past a certain point the cost for performance stops scaling linearly, like when you need to move from 4GB to 8GB sticks of RAM and they suddenly cost a ton more per GB
