[quote name='Rayeh']
I don't recommend running out and upgrading to an SSD. If your system's main HD is a 5400 rpm drive (unless it's a laptop), your system won't be able to fully take advantage of an SSD in the first place. There are smarter ways to spend your money.[/QUOTE]
The response to that is either "Not always" or "you couldn't be any more wrong"... If we're talking about a machine that's so old it only can use IDE hard drives, it most likely has an AGP video card too.. In which case your bottleneck is an ancient PC, not a single component. In any semi modern machine.. something with a SATA II port on it, SSDs will blow away any spinny metal disk in terms of both reads and writes. It is not uncommon for manufacturers to slap in the cheapest drive they can in prepackaged desktops, putting in a 5400 RPM drive saves them a couple bucks, and most people won't know any better than to care about the difference it makes.
Load times are a huge part of the experience. waiting for seconds rather than a minute or more for maps to load has a huge impact on the experience. As far as in-game performance, too much disk thrashing will impact in-game performance.. more in the campaign than multiplayer, but even so.
If I had $300 to spend on upgrades for a computer, and I had a machine will many poor components, I would rather spend $150 on a SSD and $150 on a video card than $300 on a video card. Other than extreme bottlenecks, like less than 2 gigs of RAM, or an onboard video card, a SSD *IS* the single greatest boost to speed you can put in a computer right now.