Hey Komito, I don't know about buying from amazon and ebay, best deal for you would be to go into a local shop, sit with them for 10 minutes and explain exactly what you need.
Don't buy anything in a whole package as they usually add price for no reason, while you are probably gonna get something you don't want.
If you're gonna be making maps, especially big maps, you're gonna need a little bit more RAM and better graphics card as you'll have problems using the software without it (on my older laptop, 3D rendering a single cube took around 10 minutes for example).
Also, it wouldn't hurt if you're already at ~3-4GBs of RAM to take dual core CPU, you won't need more that that, just make sure it's not AMD, as I had to learn the hard way, cheap AMDs suck ass. No matter what anyone says.
You'll pay a couple of extra bucks for Intel, but hey, you get what you pay for, ask yourself if you want to spend next 5-10 years complaining about having a shitty PC that doesn't fit your needs because you saved 30$ while purchasing it.
Anyway, just present your idea to someone who knows what he's doing and they'll help you
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