How's this (with K+M, might want some speakers too though):
Product Name Qty Price Line Total
Dell IN2010N 20" Widescreen LCD Monitor - Black £105.74
(£89.99) £105.74
(£89.99)
Corsair Dominator 4GB (2x2GB) DDR3 PC3-12800C9 1600MHz Dual Channel Kit (CMD4GX3M2A1600C9) £76.36
(£64.99) £76.36
(£64.99)
AMD Athlon II X3 Tri Core 440 3.00GHz (Socket AM3) - Retail £60.00
(£51.06) £60.00
(£51.06)
OcUK GeForce 8800 GTS 640MB GDDR3 HDTV/Dual DVI (PCI-Express) - OEM (Grade A Refurb - 3 Months Warranty) £52.86
(£44.99) £52.86
(£44.99)
MSI 770-C45 AMD 770 (Socket AM3) DDR3 Motherboard £49.98
(£42.54) £49.98
(£42.54)
Zalman Z7 Plus Midi Tower Case - Black £42.99
(£36.59) £42.99
(£36.59)
Corsair CX 400W ATX Power Supply (CMPSU-400CXUK) £32.99
(£28.08) £32.99
(£28.08)
Samsung SpinPoint F3 500GB SATA-II 16MB Cache - OEM (HD502HJ) £32.99
(£28.08) £32.99
(£28.08)
Logitech MX518 Gaming-Grade Optical Mouse (910-000616) £22.98
(£19.56) £22.98
(£19.56)
Cooler Master Hyper 212 Plus (Socket AM2/AM2+/AM3/775/1156/1366) £19.99
(£17.01) £19.99
(£17.01)
Pioneer DVR-S18L 22x DVD±RW SATA Labelflash ReWriter - Black (Retail) £17.99
(£15.31) £17.99
(£15.31)
Logitech Classic Keyboard 200 (968019-0120) £9.99
(£8.50) £9.99
(£8.50)
Sub Total : £446.70
Shipping cost assumes delivery to UK Mainland with:
DPD Next Day Parcel
(This can be changed during checkout) Shipping : £19.10
VAT is being charged at 17.50% VAT : £81.52
Total : £547.32
http://www.ubuntu.com/
Easily more than enough for the stated needs. If you really need it to come in under £500:
Product Name Qty Price Line Total
Dell IN2010N 20" Widescreen LCD Monitor - Black £105.74
(£89.99) £105.74
(£89.99)
Corsair Dominator 4GB (2x2GB) DDR3 PC3-12800C9 1600MHz Dual Channel Kit (CMD4GX3M2A1600C9) £76.36
(£64.99) £76.36
(£64.99)
OcUK GeForce 8800 GTS 640MB GDDR3 HDTV/Dual DVI (PCI-Express) - OEM (Grade A Refurb - 3 Months Warranty) £52.86
(£44.99) £52.86
(£44.99)
AMD Athlon II X2 Dual Core 250 3.00GHz (Socket AM3) - Retail £51.69
(£43.99) £51.69
(£43.99)
MSI 770-C45 AMD 770 (Socket AM3) DDR3 Motherboard £49.98
(£42.54) £49.98
(£42.54)
Corsair CX 400W ATX Power Supply (CMPSU-400CXUK) £32.99
(£28.08) £32.99
(£28.08)
Samsung SpinPoint F3 500GB SATA-II 16MB Cache - OEM (HD502HJ) £32.99
(£28.08) £32.99
(£28.08)
Xigmatek Asgard Midi Tower Case - Black £26.99
(£22.97) £26.99
(£22.97)
Cooler Master Hyper TX3 CPU Cooler (Socket 754/939/940/AM2/AM3/775/1156) £15.99
(£13.61) £15.99
(£13.61)
Samsung SH-S223C/BEBE 22x DVD±RW SATA ReWriter (Black) - OEM £13.99
(£11.91) £13.99
(£11.91)
Logitech Deluxe USB Keyboard (Black) - OEM (967738-0120) £7.14
(£6.08) £7.14
(£6.08)
Logitech S96 PS/2 Optical Wheel Mouse Black - OEM (953688-1600) £5.99
(£5.10) £5.99
(£5.10)
Sub Total : £402.33
Shipping cost assumes delivery to UK Mainland with:
DPD Next Day Parcel
(This can be changed during checkout) Shipping : £19.10
VAT is being charged at 17.50% VAT : £73.75
Total : £495.18