Battlefield 2 lag

Probably some of each but the Ram is easier to check and upgrade. If you can borrow some more ram from a friend and try it. BF2 I know likes lots of ram, when I run CS:S only 1gb of my 2 gets used, BF2 uses almost 70-80% of that 2gb.

You need 2Gb of ram to run BF2 smoothly as it takes about 1.2 Gb to run it. I had the same delima with my high performance ram 2x512Mb so I got another pair and made 2Gb ramage and never had any lagging/choppy problems again.

No. Your motherboard will almost certainly switch the 2T/1T timings and run the RAM at 333Mhz (instead of 400). It does this to ensure stability.

If you buy exactly the same RAM as you already have, you may be able to get it to run at DDR400 without problems. You'll need to manually select it in your BIOS settings. I did exactly this, and it works fine.

You know describing performance issues as lag when you're playing an online game isn't very helpful imo.

I have 1 gig of ram on my machine, everything maxed, BF2 runs perfectly.

What exactly is this "lag" you are describing? As in the game stutters, pauses and resumes or actual net lag? Are you playing online when this happens or playing against bots? In the version before the most recent patch I used to get odd game pauses mainly because of my firewall. The most recent patch resolved that problem not that I play the game with any regularlity.

It's only around 2-5% performance in difference. So get another 2x512Mb and save money that way.

Like everybody else says BF2 does use a lot of RAM so I would say thats your best bet.