I also have a 4870. 6GB ram and an E7200 @ 3.8
Everything on low I probably average 35fps so I rarely even play the game anymore. Its just how this game runs on our systems
Vista/7http://markholmes.sapphiretech.com/Drivers/Catalyst/9_12/catalyst_9.12_windows_7_vista_dec14.zip
XPhttp://markholmes.sapphiretech.com/Drivers/Catalyst/9_12/catalyst_9.12_windows_xp_dec14.zip
fixed link. slow as fuck DL
edit: heres a quick mirror (none so far for XP)...
My 2005FPW just bit the dust so I'm looking to get a new monitor... price range is $160-190 with a max resolution of 1680x1050 or preferably 1920x1200. post or send a pm! thanks :)
keep the 260 and overclock your CPU to 3.4-3.6 if youre comfortable. Those two games are somewhat CPU dependant, especially L4D2, being a Source engine game. borderlands is UT3 based (afaik) which also should not tax a 260 much.
what res do you play at? that might help explain poor performance...