How to know that a computer has PCIe x16 slot or not ?

Solution
You have one PCI-E X16 slot on your motherboard, highlighted in red below;

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Here are my motherboard specs.:
Manufacturer Dell Inc.
Model 0RF703 (Microprocessor)
Chipset Vendor Intel
Chipset Model Q965
Chipset Revision C1
Southbridge Vendor Intel
Southbridge Model 82801HB/HR (ICH8/R)
Southbridge Revision B0
BIOS
Brand Dell Inc.
Version 2.6.6
Date 6/26/2011
PCI Data
Slot Proprietary
Slot Type Proprietary
Slot Usage Available
Data lanes x1
Slot Designation SLOT1
Characteristics PME
Slot Number 0
Slot PCI
Slot Type PCI
Slot Usage Available
Bus Width 32 bit
Slot Designation SLOT2
Characteristics 5V, 3.3V, PME
Slot Number 1
Slot PCI
Slot Type PCI
Slot Usage Available
Bus Width 32 bit
Slot Designation SLOT3
Characteristics 5V, 3.3V, PME
Slot Number 2
Slot PCI-E
Slot Type PCI-E
Slot Usage Available
Data lanes x1
Slot Designation SLOT4
Characteristics 3.3V, PME
Slot Number 3
I HAVE POWER SUPPLY OF 305 WATTS ... CORE 2 DUO 2.13 GHZ , 4GB DDR2 RAM , DELL OPTIPLEX 745 E6400 MINITOWER.
 
In another thread , a guy told me that this graphic card is compatible for my pc
http://www.kaymu.pk/xfx-amd-radeon-hd-5450-2gb-ddr3-vga-dvi-hdmipci-expressvideocard-798992.html
I have 305 watt power supply 2.13 ghz core 2 duo , 4gb ram ddr2, And according to "unknownofprob" I have one PCI E X16 slot . But in newegg website . I saw in my specs. that this card needs minimum 400 watts of power supply , please help, is it fine for my pc or not? The newegg url is :
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150673