I see these RAM questions asked all the time...and for good reason...because until you really upgrade to a high level of RAM...especially if you've been at 256 or 512 for a while...it really is difficult to justify the extra $$$.
So, I'm going to say this...those of you who disagree, that's ok, because I'm speaking from Real World experience, not just what I've heard or read. RAM is like HP in a vehicle. Typically, the more you have, the better. It's late & I'm tired, so that's the best analogy I can come up with. For those of you who are into fast cars & bikes, I'm sure that it would be difficult to say..."You know, my car just has way too much HP. I really don't know what I'm going to do with all of it."
Not gonna happen. Well, IMHO, RAM is the same thing. Buy what you can afford to buy.
Now that I got that out of the way, I'll answer your question. Yes, I saw a BIG jump in performance when I recently upgraded from 512 to 1 GB. Yes, I saw another BIG jump in performance when I upgraded to 1.75 GB. Especially in BF2. I don't care what others say about RAM. I don't believe you are ever wasting your $$$ when you add RAM to your system. Just doesn't make a whole lot of sense. I'm about to go up to 2.75 GB & then 4 GB in the near future.
Remember, your system is only as fast as your weakest link. It's either going to be your CPU, Video Card or your RAM. I do however, recommend you upgrade your RAM in increments, so that you don't put yourself in a position where you can't afford a fast CPU or a good Video Card. RAM is the one component you can upgrade incrementally whenever you can afford to...a GB here, a GB there...& so on.