not necessarily. depends on what games you'll be playing. have a look at some of the cpu benchmarks here on toms or passmark. the extra core could make a difference in cpu physics heavy games like crysis.
calm down youngster...
that's bad advice in this situation.
I just put together a rig with a MSI 78GM-P45, AthlonII X4 640, 8800GT, 4 gigs ddr3, WD 1TB Green and Windows XP Pro 32. The Athlon II X4 640 stock runs Great. I have a Zalman 9700 cooler running. And oh yes, my first rig is a MSI 790FX-g70 with Phenom II 965BEC2 OC to 3700, 8 gigs ddr3, Radeon BE 5850 with Intel ssd 80g; Samsung 1 tb spinpoint 3; other rig Asus M477TD with Phenom II 965BEC3oc to 3700 with Radeon5850 BE etc. I know what Phenom IIs are like and I'm telling you the AthlonII X4 640 is a great little chip. Runs cool and runs COD BO just fine.
I have another rig with a Phenom II 720 and it is fast and oc to 4 core 3200 but for most games the 640 gives you a guaranteed 4 core chip plenty fast enough for most games.
Just keep the 740, you'll love it. The 740 is a fast 3 core with L3 and might unlock a 4th core if you care to try. I have had the 720 and recently built a 740 based rig for a friend. He added a 6850 and is in love with his configuration. Replace the 8800 when you can and you'll be very happy for a year or so.