If you really cant/dont want to build your own and want a decent gaming machine check out http://www.dell.com/xpshttp://www.gateway.com/systems/seri...s/529598059.phpor if you want something more expansive go for Alienwarehttp://www.alienware.com/product_pa...ll_default.aspxThose are all good brands but I honestly recommend building your own. There are so many reasons to do so.
Dell is not a good brand and thats coming from experience, if you want a computer that works for more than a year then don't buy from Dell. Same with Alienware which is just a subsidiary of Dell.
Also out of the choices that you presented I'd go for the second. Even though I'm not an AMD fan a lot of users are satisfied with it and it looks to be the most powerful computer out of the bunch.
Also I'm not sure about if you would need 3GB but if you did want to upgrade then the prices range from as little as about £10 to about £60-70 It's preferance of where you want to buy it really but make sure you get the correct type of RAM memory (usually DDR2 nowadays)