Not sure about the CPU but you do have 2 gigs of ram and it has PCIE x16 slot for the video card, You may not need to upgrade the cpu (although it would help some games) but may not be necessary to still play most of them. I would send an email to Gateway and get the CPU support info for your particular system and for a video card you can get a ATI 4850 that would handle any of the current games out there rather well. This one is $139 after rebate. Newegg.com - SAPPHIRE 100245L Radeon HD 4850 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card - Desktop Graphics / Video Cards
I did just do a little research on your PC and I don't think it will take any of the newer breed duel cores, yours also has the older DDR memory so I don't think upgrading would do you tons for the cpu, The video card would however do tons for playing the games and would transfer over to a new computer if you got a newer one. Sorry I didn't have better news for you, so I would upgrade the GPU (video card) and start saving for the parts for a newer PC.