When I went from dual core to quad core, I saw a difference. Especially in the Battlefield games. Check your current board though, a lot of those older AM2 and AM2+ boards can actually take an Athlon II quad core or a Phenom II quad core, which if you did that and upgraded your video card, you'd be ahead if gaming is what you want to do.
As I said though if gaming is the primary reason, see if your current motherboard will take an amd quad core if you could put an Amd quad core in and maybe upgrade to a radeon 7850. I can about guarantee you that you'd get a better performance boost in games if you go that way. Sell your old chip and video card and start saving for a new board and processor whether amd or intel.
In terms of visible day to day changes, the quadcore and a SSD would be the biggest visible difference makers. Additional RAM will help when you're multitasking or running beastly games like BF3 but it's not a game changer.
The SSD will improve load times, but it won't increase in-game performance that significantly but the CPU upgrade will help with the load and your GPU will be the bottleneck instead of your CPU.
Some games are more processor intensive (Dragon Age, larger scale RTS games, etc.) and some games are more GPU intensive (BF3, Farcry, most FPS') - what are you planning on playing mostly?