Current build what to change
 

AMD Athlon II X2 250 Dual Core Processor AM3 3.0GHZ 2MB L2 Cache 65W
MSI 870-G45 ATX AM3 870 DDR3 2PCI-E16 PCI-E1 3PCI SATA2 GBLAN Audio CrossFireX Motherboard
Crucial DDR3-1333 2X2GB 4GB Dual Channel Memory
ASUS GeForce GTX 260 896MB 448-bit GDDR3 PCI-E 2.0 HDCP SLI PhysX Video Card
Seagate 7200.12 500G Sata II 32M HDD
LG 22X DVDRW
Black OEM Case with 480W PSU

what would u upgrade and to what advice really appreciated im a noob havent had a computer since a p4 1.7

i wanna play bfbc2 and black ops really good

I would say to at least upgrade your gpu for a gtx460 1Gb or ATI HD 5850, which are 40-45% faster than the gtx260, are these aren't too expensive, nice upgrade imo. But the rest, I prefer to let more experienced people answer you.

You should be able to fit in a either a HD6850, HD6870 or a GTX460 but your processor could be a potential bottleneck for these cards and your PSU is of unknown origins.

I am wondering if that PSU is really 480W true power but you've used a GTX260 on it that requires 2x6pin PCIE power then I guess it is possible to upgrade to the GC that I mentioned above.