If you're going to upgrade your CPU, you'll want a new motherboard and new memory, too.
$220 Intel Core i5-2500K quad-core 3.3 GHz LGA1155 CPU w/ HSF
or $125 Intel Core i3-2100 hyper-threading dual-core 3.1 GHz LGA1155
$100 Gigabyte GA-Z68MA-D2H-B3 micro-ATX Z68 LGA1155 motherboard
$55 -10 code "EMCKCHB22" 2x4 GiB PC3-10600 G.Skill F3-10666CL9D-8GBRL (DDR3-1333, CAS 9, 1.5 V) memory
A GPU upgrade would have the biggest impact on your in-game experience.
$240 -30MIR XFX Radeon HD6950 1 GB graphics card
or $227½ Zotac GeForce GTX560Ti 1 GB
or $242½ Zotac GeForce GTX560Ti 1 GB AMP! (950 MHz)
or $270 -30MIR HIS Radeon HD6950 2 GB (840 MHz)
An SSD would make your desktop and web browsing much faster, but wouldn't do much for your gaming.
$200 0.12 TB Corsair Force 3 (SandForce SF-2281) SSD
or $145 -10MIR 0.10 TB Kingston SSDNow V+100 (Toshiba T6UG1XBG)
or $215 0.13 TB Crucial M4 (Marvell 9174)
or $260 0.12 TB Corsair Force GT (SandForce SF-2281)
or $275 0.12 TB OCZ Vertex 3 Max IOPS (SandForce SF-2281)