XFX Radeon HD 4870 for my build

yeah i think its worth the extra money for the 1gb version, especially if you're playing crysis.

Sounds like you have a good plan, mate.

For 1280x1024/1440x900 and you'd be better served with a HD4850 or GTS250.
If you play at 16x10 I'd say stick with the HD4870 512 or GTX260, both are plenty for that but at 19x10 or 19x12 look towards the GTX275/HD4890.

Just a few points: All these cards are quite high performance, so if you use the system in a hot room, make sure the cooler exhausts its heat out of the case or you could have overheating issues.
Make sure your PSU is up to the job and has enough PCI-E 6 or 8 pin connectors. Try to avoid using the molex/PCI-E adaptors. I suggest you change the PSU rather than use adaptors for the GTX275/HD4890.
Do n't worry about the PCI-E version; As long as your current motherboard has a PCI-E X16 slot and the card will fit the case, you're good to go.
All the cards listed will be restricted by your current CPU, I'm making these suggestions based on the CPU/MB/RAM upgrade being large enough to eliminate the restrictions, not your current setup.

Thanks for pointing out the power source, I'd overlooked that. My PSU only has one six-pin PCI-E connector, and this card requires two. It does say it comes with a six-to-four pin power cable, but it sounds like you're suggesting that may be problematic? I'd rather not have to get a new PSU right now as well.
EDIT: No, I got that wrong, it has two, so I should be good.

Just to satisfy my curiosity: Exactly which PSU do you have? Make and model, please, because not all powersupplies are created equal, and 500W for one is not the same as 500W for another.

That's actually another question I have, about the memory. My monitor is 1280x1024, and I'm happy with that and have no plans to change it.

I'm pretty much decided on a 4870 or a 4850, but now I want to know which version I should get. There are a few models available at the site I am buying from, and I'm not sure what some of the differences are, or how much difference the built-in RAM makes. When I was last putting a build together, the advice I got was that GPU RAM is actually not a terribly important factor, as long as you have enough RAM in your build. So is there much difference between the 512 and the 1 gig versions of this card? And what does he suffix "xxx" mean? I've tried looking it up but had no joy.

This is the site I'm looking at: http://www.edbpriser.dk/Product/Details.aspx?pid=489855

It's in Danish, but the product info is all in English, and if you scroll down a little you can see the three variants available under "relaterede produkter" (a chocolate fish for the first person to guess what that translates to). Danish kroner are about 5-6 to one for USD, or 8-10 to one for GBP, so the difference between the most and least expensive versions of this card is about a hundred bucks, but really I'm more interested in the two cheaper ones. A 512Mb, or a 1Gb XXX for about 30 dollars more?

How's about related products ?? What did I win again ??

I wouldn't buy a 512 for myself any more. If you think saving the $30 over better "performance" is ok, do it. It's your machine.

I think having more ram ( system ) is good when there's enough to keep your hard drive from getting dumped on..... which causes stumble or stutter while gaming. More ram on your video card will allow you to turn up the settings for a better visual display while gaming.

it actually depends on the size of your monitor . . . .. . . and if you will do overclocking . . .i guess you need a much better cooler like coolermaster HAF 932 for more air flowing

Right, but what I'm asking is, does the RAM have to be in the GPU? I don't want the computer resorting to swapdisk space no, but that won't happen so long as I have enough conventional RAM, and it seems like you can get a lot of RAM very cheap at the moment. Is there a difference between say 3 gigs total RAM with a 1 gig card, and 3 gigs total with a 512mb card? (overlooking the fact that having 2.5 gigs of conventional RAM would be unlikely and perhaps inefficient in itself). Do GPUs use their own RAM faster or better in some way than general RAM?

@ Jurosem: I think you know this but I'll mention it any way.
A 32 bit OS can address 4Gb of memory in total and Windows allocates those addresses to add on cards and peripherals first, whatever is left is used by Windows. So, if you have 4Gb of installed RAM and a 1Gb video card, windows allocates 1Gb(plus some overhead) to the card, leaving less than 3Gb for itself.
My rig can use 3.25 Gb out of 4Gb of installed RAM by the time Windows has allocated space to the sound and graphics cards.
A 64 bit OS can address Terrabytes, so the issue does not exist there!

Normal RAM stores the program and as much texture and geometry data as possible so you need as much as is reasonable to prevent the swap file being used.

The video RAM is used exclusively by the card itself and is fed data from the CPU, (mainly raw geomenty) for each frame to be rendered, it also holds texture data which is moved from/to system RAM as required.
The video RAM is also where the card constructs the 3d image, lights it, renders it and applies filters such as AA and AF.

As for system RAM: I'd go for 4Gb as 2X2Gb sticks: It saves messing around with a 2X1Gb+2X512MB configuration for 3Gb, can actually be cheaper and is less likely to have stability issues.

As for the card memory: I have never had any problems with a 512Mb card but then again I use very little AA but if the 1Gb card is only $30 more it would make a wise investment.

Thankyou for the detailed reply Coozie. No I didn't know that stuff. Not to be a pain in the arse, but what exactly does that mean to the specific question of how much the RAM in the card matters? As long as you have enough total RAM, for the card and everthing else, does that mean that a 1 Gb card won't be any faster/higher performing than a 512Mb card? (Assuming they're otherwise identical) Or does it slow things down if the card has to draw on RAM external to itself? I'm guessing that's unavoidable anyway, unless you have a card with a lot of RAM and/or very light load on it.

This is a bit off-track, but as I have 2x1Gb RAM now, it would be nice if I could just add either 2 more 1Gbs, or one 2Gbs. I seem to recall hearing that RAM works better in parallel though, I think I may even have read that 2x1 will (at least in some cases) run faster than a single stick of 2gb. I aso heard that (some, most or all?) mobos that have 4 DIMM slots actuallu don't like it if you use them all, and will slow down if you occupy all four.
I'm not actually buying RAM right now anyway, but these answers would be useful for when I do  

The extra ram on the video card does not make it run faster it enables you to have more going on... AA/AF, etc. and less "swap out". The only way a card is going to be faster is to buy the fastest card you can afford.... see post some where above about the few extra frame you'll get.
The memory.... drop the idea of mixing different ram and speeds and quantity. Buy 4gig ( 2-2gig sticks ) as last poster above mentioned.
Read this. I'm sure the values have changed a bit since this came out...

http://www.yougamers.com/articles/13801_video_ram_-_how...

Playing at 12x10 go with 512mb.

Cool, I've gone ahead and ordered the 512Mb 4870. It seemed the best value for money for what I want. When I eventually upgrade the RAM I'll go with your suggestions. Thanks guys.