i have the e6400 and ds3. but i have 2 gigs of corsair ddr2 800. After rebate, you can get the 5-5-5-12 version of it for about 230, which is just a little over your budget, and it also means waiting for the rebate. If by space you are talking about how many gigs you want, I recommend 2 gigs if you are a heavy game user. If you are just a light game user and multitasker, 1 gig is plenty. But i do recommend doing dual channel, which requires 2 identical sticks installed at once. In terms of speed, if you aren't going to overclock much, i would just get some decent ddr2 667. that will allow you a decent overclock, while not busting your wallet.
and as for a e6300 or e6400, its a $40 difference. You can easily get a quick overclock without really trying on either one of those. they are both pretty damn good, once overclocked, better then most amd stuff which was top of the line just a few months ago. if $40 bucks matters to you, just go with the e6300 and overclock it to 333*7 which gives you 2331 and keeps your ram in a 1:1 ratio if you get the ddr2 667. for a total of maybe 100 dollars more though, you could probably get ddr2 800 and the e6400. maybe even 2 gigs of ram depending on what brands you're looking at. wow. i wrote a lot so i'll let that sink in for now. just write back if you have any more questions.