Why is my Mac not properly running steam games?

-I downloaded Civilization 5. There was a serious amont of "lag" or whatever you call it when a singleplayer offline game slows down. But it was Civ 5, which has a lot of stuff going on at once, so I figured that was just normal. And at least it was playable on smaller maps with less nations.



Then I got Team Fortress 2. It's more or less unplayable with my internet connection. The lag was unbearable. But whatever, right? I never expected my wi-fi to handle it.



Then I got Splinter Cell Conviction, a game that ran fine on my Xbox 360, but which I no longer have. I figured it would save me a trip to the store, so I got it on Steam instead... I can't even get past the main menu. The mouse lags on every option. But it's offline singleplayer. So it can't be my internet, right? And my computer CAN open the game (it just kind of breaks it through a lag tsunami)... so it's not system requirements... is it? Well, I guess it has to be...



(this is why I have a 360. No system requirements there. No installation. And immersion in using a controller versus the mundane point-and-click of a computer.. no contest. But let's avoid that argument.)



But I digress.



Here are the specs (or what I think specs are), if you need more, let me know. I pretty much know nothing about computers.



Model Name: iMac

Model Identifier: iMac7,1

Processor Name: Intel Core 2 Duo

Processor Speed: 2 GHz

Number Of Processors: 1

Total Number Of Cores: 2

L2 Cache: 4 MB

Memory: 1 GB

Bus Speed: 800 MHz





ATI Radeon HD 2400 XT:



Chipset Model: ATI,RadeonHD2400

Type: GPU

Bus: PCIe

PCIe Lane Width: x16

VRAM (Total): 128 MB

Vendor: ATI (0x1002)

Device ID: 0x94c8

Revision ID: 0x0000

ROM Revision: 113-B2250A-207

EFI Driver Version: 01.00.207

:Hahaha, ur Mac will be lucky to run plants vs zombies. You need better everything. Most important is more ram, larger hardrive, more speed (GHz), better graphics card. Macs arent really meant for gaming. Either just play on xbox, or go buy a PC.
It's your graphics card,

I have the same mac, and find it hard to play most games.