You can't directly connect the MacBook to speakers without it sounding worthless because it doesn't have a line output. There are some other decent options, so if you could give us some more info about the setup, hopefully we can give you good advice.
The best and cheapest thing to get this working may be a receiver with at least two TOSLink connections, but I'll just let you tell us a bit more before I go overboard.
Most importantly: do you want surround sound, or stereo, and what is your budget?
Edit: Without having more info, here is my best guess for a good setup for you. First off, if it's not a problem to always have the MacBook connected to the PS2, get an EyeTV Hybrid and a cheaper monitor. What will be nice about that route is that you will be able to control the volume of the Playstation with a remote, instead of having to manually "dial in" the level.
That Dell doesn't even have component inputs. However, if you need more freedom, go ahead and get a monitor with more NTSC-related connections. It looked like the Dell in question had an S-Video input, so by all means use that instead of the composite input if you go that route. I use the following excellent cheap monitor with an EyeTV Hybrid and my Wii:
http://www.newegg.com/Product/Product.aspx?Item=N82E16824112007
Here's the cheapest way to get a line level signal out of the MacBook:
http://www.sweetwater.com/store/detail/iMic/
Then, here are some nice moderately-priced audio monitors that have an RCA and a 1/8" stereo input:
http://www.sweetwater.com/store/detail/MA7A
or
http://www.sweetwater.com/store/detail/MA7ABK
And here is the better version, but I don't know the budget yet:
http://www.sweetwater.com/store/detail/MA15D/
or
http://www.sweetwater.com/store/detail/MA15DBK
I was also just thinking that you may not even need the iMic, because these speakers have TOSLink inputs, and the MacBook supports TOSLink out. However, I can't vouch for how encoded surround sound is handled from DVD Player and the like, when you don't actually want an encoded stream. Perhaps someone else has relevant knowledge?
