To a certain extent, the amount of memory you have doesn't impact how many "bounces" you'd need to wait for. To understand why, I'll try to illustrate what each of these components actually do. Memory (RAM) holds information that the computer is actively working with. When you open a program, it gets loaded into the RAM. But where is it loaded from? The hard drive. When you're watching the icon bounce on the dock, what's happening is the hard drive is actively seeking that program's information and is then transferring it to the memory. When the program is fully loaded, then you know that the hard drive has finished finding all of the program and has loaded it into the memory. However, occasionally the program may pause for a few moments while you're using it. One frequent cause for such a thing is that the program is again waiting on the hard drive, perhaps because an aspect of the program wasn't loaded into the memory (or was unloaded) or because the hard drive needs to find the file that the program relies on for its next function.
The reason I started this explanation off by saying "to an extent" is because when it comes to performance and system responsiveness, there are a few other operations to understand.
The first is that memory is actively managed by the computer, and things are loaded in and taken out. Open a program, quit it, and then open it again: you'll find that the second time you open it, the load time is much, much faster. This is because the computer did not remove everything from the memory when you quit the program, and thus there was less for the hard drive to copy back into memory. This happens as a time-saving measure, and it works nicely. But if you're doing a lot of things then the computer may be forced to unload even more from the memory, which brings you back to square one. Having more memory allows the computer to keep more unused items in the memory, but this does nothing to speed up that initial load. You'll always need to get information from the hard drive to the memory in the first place.
The second consideration is what happens when you're doing so much that even all of your active information can't fit in the memory. In that case the computer creates something called a "swap file," which is a file on the hard drive that the computer knows contains data that it still needs. The computer then copies data from the memory to the swap file, and from the swap file to the memory. When this happens things tend to slow down dramatically, because the traditional hard drive is such a bottleneck in computing speeds. Buying more memory helps with this issue only because it makes the reliance on the swap file - and your hard drive - that much less.
The final consideration is the way that the hard drive works. I mentioned previously that there's a little arm with a reading head that moves across a spinning platter. Aside from having to physically move to find things, the head can't be everywhere at once. Even if you're not doing anything, the operating system is completing background tasks that require hard drive activity. Listen to a MP3, load a program, and have a download running: all of these are adding demands to what the hard drive must do. Hard drive technology has come a very long way from what it used to be, but one reading arm and platter can still only go through one task at a time.
A solid state drive (SSD) doesn't have the physical limitation mentioned above, and it's much faster in every form of operation.
If you're attempting to purchase a current-generation computer, then the cost of the computer doesn't fluctuate much no matter where you get it from. The real cost savings come from buying systems that are one or two generations old. They're still new and in their original product boxes, but the discounts there can be in the hundreds of dollars. While in college, I bought an older MacBook from my campus bookstore with such savings; more recently, I've used MacMall for the same purpose. The best time to find such deals is right after Apple announces refreshed products. You might be able to find some if you look now, but most of the systems have probably already been purchased and as a result the selection is likely to be very limited.
Regarding memory and other components, such as external hard drives, people have different preferences. I've been using NewEgg.com for over a decade at this point. Their sale prices tend to be better than what's found on Amazon, but I've noted at times that Amazon has the better price.
Regarding memory, the whole "Mac compatible" thing is a scam, in my opinion. You can use the same RAM (memory) as PCs do. Make sure that the type is correct (how many pins - it looks like iMacs use 204-pin RAM, or "laptop RAM"), that the specifications are correct (nearly everything is DDR3 clocked at 1600 MHz now, but when DDR4 becomes the norm you'll need to pay closer attention), and you're set. While some brands are considered more trustworthy than others, I don't think there's such a thing as "bad RAM" these days, unless perhaps you're buying grey-market RAM from eBay. Crucial and Corsair are big names; Mushkin has established itself; Kensington is another big name, but without "performance" lines; G.Skill and GeIL (my personal preferences) cater toward performance but also have some "economy" offerings... and there's a lot more.
