Will Team Fortress 2 run on my pc(?_?)

Ram is usually most important.. you sometimes can't run a game if you don't have the minimum ram space required.  Because the difference of your ram with whats required isn't to great it may run but its probably going to run like crap (You should check to see if steam has a demo you can try out)

How did you end up with 504mb of RAM? Seems like an odd amount...
 
Anyway it should run, the Source engine is pretty well optimised so just throw it on lowest settings and you'll be fine.

Buy some memory. Its so cheep not too. Most likely you have the weird amount because of integrated graphics which if your not using, turning it off will give you back that 8MB.