Loads of good advice here - Answerbamk's great for this.
More advice - don't go for the latest cutting edge machines - they're usually overpriced. Read reviews - some manufacturers assemble the machines better.
See what Amazon are selling and what's most popular in your price range. I bought a cheapish (£300) Zoostorm which was their number one best seller, and it's been better than the previous one - a Dell costing around £1000.
Lastly, never listen to anyone in the High Street computer shops.
Good hubting!