Media center questions

I got a demension 8400 last year.  It has MCE which I have not used yet.  I got a remote kit that has a remote and a little black box that you  plug into a usb and then 2 separate holes I am not sure what you plug into but it has a little wire with it.  Anyway, is that the tv tuner?  I know my computer has a spot in back to plug a tv cable into and it is audio/video in and out.  I noticed on this fourm a long time ago I asked about the media center extender.  I got the wireless router someone told me to get that will work with the extender,  My computer has....3.2 ghz, 1 GB ram, , media edition 2002 with service pack 2.  I am not sure what else you would need to know about the computer. 

 

What is the extender for?  What EXTRA's do I need to hook everything up?  Which extender should I get?  As I said I have the wireless router I was told to get.  I have regular cable, not dish or satalite.    i am ordering my son a new computer for Christmas.  this is what it is...

 

Dell Dimension E510 SeriesIntel® Pentium® 4 Processor 630 w/HT Technology (3.0GHz,800FSB)Operating SystemGenuine Windows® XP Media Center 2005 EditionMemory1GB Dual Channel DDR2 SDRAM at 400MHz (4x256M)KeyboardDell USB KeyboardMonitorFREE UPGRADE!! 19 inch E193FP Analog Flat PanelVideo Card128MB PCI Express™ x16 (DVI/VGA/TV-out) ATI Radeon X300 SE HyperMemoryHard Drive160GB Serial ATA Hard Drive (7200RPM) w/DataBurst Cache™Floppy Drive and Media ReaderNo Floppy Drive IncludedMouseDell® 2-button USB mouseNetwork CardIntegrated Intel® PRO 10/100 EthernetModem56K PCI Data Fax ModemAdobe SoftwareAdobe® Acrobat® Reader 6.0CD ROM/DVD ROMSingle Drive: 16X CD/DVD burner (DVD+/-RW) w/double layer write capabilitySoundIntegrated Audio with Dolby Digital 7.1 capabilitySpeakersDell A215 SpeakersOffice Software (not included in Windows XP)Microsoft Office Basic - Includes Word, Excel and Outlook emailAnti-Virus/Security Suite (Pre-installed)McAfee SecurityCenter with VirusScan, Firewall, Spyware Removal, 15-monthsHardware Warranty1Yr Ltd Warranty, 1Yr At-Home Service, and 1Yr HW Warranty SupportDial - Up Internet Access6 Months of EarthLink Internet Access IncludedMiscellaneousAward Winning Service and SupportFinancial SoftwareNo QuickBooks package selected- Includes limited use trialOperating System Backup & RecoveryPC Restore recovery system by SymantecTV Tuners and Remote ControlsSingle TV Tuner + Remote Control - watch and record TV on your PCDell Digital EntertainmentStarter Entertainment Pack - Basic digital Music, Photo and Game experience

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Is there any need to get him the tv tuner too since I already have one (If that is what the little black box is)?  I don't expect to use both at once but would that work the same as a dule tuner?  I do believe my DH will be using the new computer too so he might want it.  Would it be ok to get 512 mb and 80 gb instead of what is listed?  I will be putting video from digital video camera into my computer and I want the tuner so I can watch and record tv on my computer.  If I am watching tv on my computer, will it interfere with the vcr or dvd on the regular tv?  Thanks