This computer is just something I threw together in a few minutes. I am not endorsing it in any way shape or form. I simply went to Dells website because they have a good selection and their systems are completely customizable.
Dell XPS 630 No monitor, keyboard, mouse or speakers.
Samsung 2033SW Monitor Personally I love Samsung monitors but that's up to you.
The keyboard and mouse you can get dirt cheap from just about anywhere, same with speakers.
If you want to do some searching yourself here are a few things to look for.
Intel processor, dual core, 2.5ghz or better. Avoid quads, very few games are optimized for them and they can actually cause a loss of performance (minor loss) with non optimized software. If you're not looking at doing any high end video editing or hardcore multitasking, it's not necessary. While I'm a huge AMD fan, Intel has completely dominated the market over the past several years.
Memory, DDR2, 4-6 gig, I suggest 6gig. Avoid DDR3, it's pricey and overkill for what you're looking for.
Video card, Nvidia 9800 series or better, at least 512mb but I suggest 1gig if you don't want to have to upgrade in the near future. In terms of video card memory, DDR3 is ok. SLI is nice but since video cards are one of the more pricey parts, a single good card is going to be more cost effective than two decent cards. Since you're ordering one and not building it yourself I won't get into AGP vs. PCI-e and power requirements. I haven't looked into video cards in awhile so I can't make any specific suggestions other than go with Nvidia over ATI.
Hard drive, go with 7200 over 5400, suggest 500gig, 10,000RPM and SSDs are overkill for your needs. By no means a deal breaker here. If you have to pickup a second HDD later on it's no big deal since they are usually pretty cheap.
Optical, at least a DVDrw, Blu-Ray is purely a matter of preference. Based purely on price, I would pass.
That's pretty much it. I wouldn't choose a keyboard or mouse if you're given the option. If it comes with it, great, if not, go buy them from best Buy or something, same with speakers.
I would also pickup a monitor separately. You can find great monitor deals from sites like Newegg and TigerDirect, both of which I have ordered from on several occasions and never had any problems.