The USA is a big place...there are plenty of areas that have climate similar to the UK.
The general recommendation is an intake fan in the lower front & exhaust fan in the upper rear, below the PSU. And the PSU should have dual fans...an intake on the underside & exhaust in the rear.
Why did you choose the Sempron 3400+ & what motherboard are you getting? I realize pricing is different in the UK, but here's a few socket 754 CPU prices to consider:
Sempron64 2800+/1.6GHz/256 L2 -- $75.50
Sempron64 3100+/1.8GHz/256 L2 - $100.00
Athlon64 2800+/1.8GHz/512k L2 - $112.00
Athlon64 3000+/1.8Ghz/1MB L2 -- $116.00 (mobile)
Sempron64 3400+/2.0GHz/256 L2 - $125.00
The S754 system is AMD's budget setup, but since you'll be buying a PSU, CPU & new motherboard anyway, why not get a PSU with a 24-pin main plug & build a S939 system instead of S754? The S939 Athlon64 3000+/1.8GHz/512k L2 (Venice core) sells for about $133.
And when you buy your CPU, buy the retail model & use the stock AMD HSF.
R.I.P. Eddie Guerrero