I found crucial DDR to be the safest option, and if you can afford it buy them registered, it may be that bit more but you get a touch more speed out of them.
Well seeing that you're only playing games, 512mb is more than enough, only reason I opted for 1GB was because, like someone's said earlier, I do heavy multi-tasking AND I use Photoshop with pics even higher than 600dpi and with more than 150 layers (roughly 4k+x4k+ pixels in size).
Oh and lol about Sim City 4, it even runs fairly slow on mine, I really need to change this GF4 MX440. I think SC4 is a GFX Card intensive game like most games nowadays, seems like RAM doesn't make much of a difference anymore, same with the processor.