My specs are horrible. LOL I have an old hp computer. I have a pentium 4 CPU at 2.53ghz, and 1gb of ram. Computer is maxed out.
Hopefully I can upgrade soon, but will I be able to play the game?
I would say no you can't because you don't meet the minimum requirements, your CPU is clocked too low.
If you get a new mobo and CPU (you can get a cheap AM2+ bundle, maybe $100 or so) then you should be able to run it no problems. Memory upgrade wouldn't hurt either.
If you do upgrade stuff rather than the whole system, make sure your power supply will handle the upgrades. If it is a prebuilt system, it will just about certainly not handle it because the power supplies they put in there are cheap, asian piles of crap that just about power the system they ewre initially put in, but not any more
i can't even get it to work on my system in the first place
I prefer just trying or better yet, reading the minimum and suggested specs OR getting it, if it doesn't work, do a bit of overclocking
I believe you can get an AGP version of the Radeon HD 4650 - though it still requires a 6-pin PCI-E power connector. not a problem if you have 2 spare molex plugs and an adapter.
The 4650 is a massive upgrade from the 7600 (well, in power consumption atleast) it draws a hell of alot more power. Because of this, his PSU may not be up to the standard, so should be looked into too.
and bare in mind also that the 4650 has about identical specs in games and that to the 8600gt, which draws very little power and requires no PCI-e connectors, but will most probably not run l4d on full settings at full AA too. But is cheap today, I don't know about in US, but in UK I could sell mine for £20 each atleast (you can only get used now, they aren't made any more)