I'm getting a new PC delivered sometime next week and I originally wanted to put my old 2GB RAM in it. Thing is the new one comes with two 1024 sticks, but my old one had 4 512s. Now I Think I remember reading somewhere that the Sims doesen't play so well with different size RAM sticks. Is that true?
Second how much improvement if any will I notice if my current card is a Geforce 7800 GTX with 256 MB and the new one is a Geforce 8800GT with 512 MB?
Last one, I swear.
How good is a Q6600 Quad Core 2.40 Ghz compared to a Singlwe core 3.4 Ghz? Any info would be appreciated.

It's not the Sims specifically that is going to matter when it comes to RAM, but the RAM itself, and the motherboard of your computer. My computer only takes twin pairs of RAM sticks - if I put in two mismatched ones in a pair of slots (I have four slots) then it will use whichever one has the lower bus speed, ignoring any benefits that the better stick of the two might have. Assuming your new one has two extra slots and is the same type of RAM (DDR2 vs. DDR), even if you're stuck doing paired sticks as I am, assuming you have 4 of the same type of 512s you should be able to put those in just fine and get the full speed out of them. You'll want to find the exact specs on the bus speed of all your RAM sticks available (including the new ones) as that, as well as the size of the individual sticks in megabytes, is going to be a big factor. If your new RAM has a much higher bus speed than the old ones, it's possible that you might do better with 1.5 gigs of ram with 1 gig at a much higher bus speed than at 2 gigs with all your bus speeds lower.

As for that second question - with Sims 2, probably not that much improvement with those two graphics cards. A 7800 GTX 256 should be able to handle just about anything the game can throw at it. Certainly an 8800 GT 512 is much better, but the 7800 can probably handle just about anything the game can throw at it anyway, so if Sims 2 is all you play, it may not be worth it to upgrade right now.

That last one I'll leave for someone else to answer as I'm not too sure about it.

Thanks for your answer. As for the RAM all six sticks are exactly the same except forthe 1024-512 size difference. The pairs thing isn't a problem. I only have pairs anyway.
The only other game I play is NWN2 which hates dual/quad cores but can be forced to run on them. It also ran perfecctly fine with my 7800 Gtx.
The whole new computer thing wasnt even my iidea to brgin with, my mother just wants my current one, because hers (3Ghz, 1GB RAM, Radeon x300 128MB) is too slow for playing Patience an Solitaire... So she's buying me a new one.