Well if you need everything including monitor and keyb+mouse this is about the cheapest you can get really
MY-030-GL GeIL 1GB (2x512MB) PC5300 667MHz Value DDR2 Dual Channel Kit (GX21GB5300DC) (MY-030-GL)
£79.99 £79.99
HD-023-SA Samsung SpinPoint P HD080HJ 80GB SATA-II 8MB Cache - OEM (HD-023-SA)
£25.95 £25.95
CD-079-SA Samsung SH-S162L 16x16 DVD±RW Dual Layer LightScribe ReWriter (Black) (CD-079-SA)
£16.49 £16.49
CA-052-AN Antec NSK4400 Mini Tower Case - 380W SmartPower PSU (CA-052-AN)
£43.99 £43.99
MB-149-AS Asus M2NPV-MX (Socket AM2) PCI-Express DDR2 Motherboard (MB-149-AS)
£44.99 £44.99
CP-162-AM AMD Athlon 64 3000+ 1.80GHz (Socket AM2) - Retail (CP-162-AM)
£44.99 £44.99
MO-016-OK OcUK Gamer Extreme 178GP 17" LCD Monitor - Black (MO-016-OK)
£91.99 £91.99
KB-072-LG Logitech Cordless Desktop EX-110 - Retail (KB-072-LG)
£17.99 £17.99
Subtotal £366.38
VAT £64.12
Total £430.50
Should do what she wants easily enough
If you didnt need the monitor, keyb+mouse, you could get this conroe rig :
MY-030-GL GeIL 1GB (2x512MB) PC5300 667MHz Value DDR2 Dual Channel Kit (GX21GB5300DC) (MY-030-GL)
£79.99 £79.99
HD-023-SA Samsung SpinPoint P HD080HJ 80GB SATA-II 8MB Cache - OEM (HD-023-SA)
£25.95 £25.95
CD-079-SA Samsung SH-S162L 16x16 DVD±RW Dual Layer LightScribe ReWriter (Black) (CD-079-SA)
£16.49 £16.49
CA-052-AN Antec NSK4400 Mini Tower Case - 380W SmartPower PSU (CA-052-AN)
£43.99 £43.99
CP-126-IN Intel Core 2 DUO E6300 "LGA775 Allendale" 1.86GHz (1066FSB) - Retail (CP-126-IN)
£114.99 £114.99
MB-004-AK Asrock Conroe945G-DVI (Socket 775) PCI-Express DDR2 Motherboard (MB-004-AK)
£45.99 £45.99
Subtotal £327.40
VAT £57.30
Total £384.70