What is the difference between core 2 duo or core i3/5/7...?

This is a choice I have on a customisable gaming laptop and im unsure of what it means and whether it is worth more money.

also are these good specs for a gaming laptop as these are what i have in mind

COLOUR CHOICE Alienware M11x - Cosmic Black edit

PROCESSOR Intel Core 2 Duo SU7300(1.3GHz,800MHz,3MB) edit

OPERATING SYSTEM Genuine Windows 7 Home Premium, 64bit, English edit

SERVICES AND SUPPORT 1Yr Next Day Hardware Support included with your PC edit

MEMORY 4096MB 800MHz Dual Channel DDR3 SDRAM [2x2048] edit

HARD DRIVE 500GB (7,200rpm) Serial ATA Hard Drive edit

BLUETOOTH Dell Wireless 365 Bluetooth Module - European edit

EXTERNAL OPTICAL DRIVE Optical Drive : 8X DVD+/-RW Drive edit

SECURITY SOFTWARE No Antivirus Software edit

Accessories

AVATAR Alienhead 3D edit

PRE-INSTALLED GAMES Steam Gaming forum and download application edit

Also included with your system

WIRELESS CONNECTIVITY Dell Wireless 1520 Half Mini Card (802.11n)

LCD 29.5cm (11.6") WideHD 1366x768 (720p) LCD

PRIMARY BATTERY 8-cell 63Whr Lithium Ion battery

GRAPHICS CARD 1GB GDDR3 NVIDIA GeForce GT 335M

Gedis Bundle Reference N01W1103

Order Information Alienware M11x Order - UK

Shipping Documents English Documentation

Power Supply 65W AC Adaptor

Cables UK 220V Power Cord

Keyboard Internal UK/Irish Qwerty Keyboard

AutoMatic Updates AutoMatic updates - On

Standard Warranty 1 year Next Business Day Hardware Support included with your PC

Hidden NamePlate Engraving Generic Name Plate Engraving

WALLPAPER Alienware 3D Wallpaper