Ok here is a comparison for you mate:
Pre built system: £1349 (£1585.08 Inc VAT)
Ultima" AMD Athlon 64 X2 4400 ATI X1900 XT-X Dual DDR System (FS-022-OK)
System built by yourself:
CP-127-AM AMD Athlon 64 X2 Dual Core 4400+ (Socket 939) - Retail (ADA4400CDBOX) (CP-127-AM) 1
£279.95 £279.95
FG-000-AR Arctic Cooling Freezer 64 Pro CPU Cooler (Socket 754/939/940) (FG-000-AR) 1
£12.95 £12.95
MB-129-AS Asus A8R-MVP Crossfire (Socket 939) PCI-Express Motherboard (MB-129-AS) 1
£64.95 £64.95
MY-079-CS Corsair 2GB DDR XMS3200C2PT TwinX (2x1GB) CAS2 (MY-079-CS) 1
£109.95 £109.95
HD-046-WD Western Digital Caviar SE16 250GB 2500KS SATA-II 16MB Cache - OEM (HD-046-WD) 1
£55.95 £55.95
GX-087-AS Asus ATI Radeon EAX-X1900 XT-X 512MB GDDR3 AVIVO TV-Out/Dual DVI (PCI-Express) - Retail (GX-087-AS) 1
£339.95 £339.95
CD-033-PL Plextor PX-755A 16x16 Dual Layer DVD±RW ReWriter - (Black/Beige) - Retail (CD-033-PL) 1
£43.95 £43.95
SC-032-CL Creative Sound Blaster X-Fi Fatal1ty FPS Edition - Retail (SC-032-CL) 1
£114.75 £114.75
HD-001-OK OcUK Value Floppy Drive - Black (HD-001-OK) 1
£5.95 £5.95
CA-037-CM Coolermaster Wave Master - Silver (No PSU) (CA-037-CM) 1
£69.95 £69.95
CA-009-TG Tagan TG580-U15 580W ATX2.01 Easycon SLi Compliant Modular Silent PSU (CA-009-TG) 1
£77.95 £77.95
OS-002-MS Microsoft Windows XP Professional Edition inc. SP2 - OEM - 1Pk (E85-04026) (OS-002-MS) 1
£79.95 £79.95
Subtotal £1,256.20
Shipping (City Link Parcel Next Day (Delivered Mon-Fri)) £12.95
VAT £222.11
Total £1,491.26
So you save around £600 quid which is nice. Oh couldn't find a black coolermaster case but u could always find one elsewhere or change your components to silver your choice. Didin't know if you wanted monitor or keyboard etc. I would also probably go for a Seasonic PSU as they are very good. I obviously didn't know how much storage you would need, but personally I would choose this instead of the WD 250GB I have quoted:
REFERENCE DESCRIPTION QUANTITY PRICE COST REMOVE
HD-015-WD Western Digital Raptor 74GB WD740GD 10,000RPM SATA 8MB Cache - OEM (HD-015-WD)
£93.50 £93.50
HD-070-WD Western Digital Caviar SE16 320GB 3200KS SATA-II 16MB Cache - OEM (HD-070-WD)
£73.95 £73.95
Subtotal £167.45
Shipping (City Link Parcel Next Day (Delivered Mon-Fri)) £8.00
VAT £30.71
Total £206.16
May add a little to your overall system but think its well worth it.