300 pounds= 525.66USD
in that case, this is what i've come up with.
if you want to buy monitor, OS, periph. then just go buy one from a department store or something, some sempron with everything included. If, however, you only need the system, then this is what i've come up with.
all from newegg:
ECS RS480-M V1.0 Socket 939 ATI Radeon XPRESS 200 Micro ATX AMD Motherboard - Retail $62.99
AMD Athlon 64 3200+ Venice 1GHz HT Socket 939 Processor Model ADA3200BPBOX - Retail $166.00
XFX PVT43GNDF3 Geforce 6600GT 128MB GDDR3 PCI Express x16 Video Card - Retail $119.00
CORSAIR ValueSelect 1GB (2 x 512MB) 184-Pin DDR SDRAM DDR 400 (PC 3200) Unbuffered Dual Channel Kit System Memory - Retail $67.98
HITACHI Deskstar 7K80 HDS728080PLA380 -0A30356 80GB 7200 RPM SATA 3.0Gb/s Hard Drive - OEM $50.00
Antec Solution SLK1650B Black Steel ATX Mini Tower Computer Case 350W Power Supply - Retail $62.99
total: $530.96
so see if you can get those parts for similar price from where you live. i don't know what taxes are like where you live, but obviously the higher the taxes the more you'll pay. if you need a dvd-wr, monitor, OS, or anything else, then you'll definitely have to downgrade some parts.
good luck