NewEgg is the best I find. Very good customer service and selection. You can also check www.pricewatch.com to see what other companies are charging as well. All reputable companies in the States have a report with the Better Business Bureau -- so if in doubt whether a company is good or not, you can check how customers feel about a particular company at www.bbb.org -- they search companies by address, phone number or web address -- very useful tool.
Not exactly the USA, but, we've bought a few MP3 players from these guys in the UK.... http://www.advancedmp3players.co.uk/shop/ They have good after sales service too. Check your Euro-Sterling budget on www.xe.com
B & H photo in New york have an online shop and they ship to Ireland as well . Even with the shipping charges most of these things are still cheaper than here. I have not bought online with them but have bought instore.
I have purchased a couple of items from http://www.bhphotovideo.com in New York and had them sent to ireland. shipping is expensive. If you have relatives in the USA its cheaper to have them shipped to them.
Can't wait to go back shopping, found another useful site with some electronic stores listed on it [broken link removed] but i'm not sure if they ship to Ireland?