Upgrading AMD 4870 512MB, budget ~£150-200

Just got myself a copy of Deus Ex: Human Revolution. Game won't even let me load it up when I turn on antialiasing or SSAO, says I don't have the required resources, and I do get some jerkyness and slowdown occasionally at 1920x1200 even with them off, so it got me pondering a graphics card upgrade (I've got a 24" monitor that this is the native res for, and I prefer to turn off stuff like AA and keep the native res than drop the res but keep extra prettyness). I'm also going to be wanting to play Rage and Battlefield 3 when they come out at reasonable framerates (N.B. I consider a steady 30fps to be perfectly playable. As long as it doesn't drop below that to 20 or so on a regular basis. I'm not one of these people who wants a constant 60+fps to be happy). I built the current system in mid-2008 and it was pretty high end back then, but I just don't have the disposable income now that I did back then so I can't afford to go building myself a new core i7 or similar.

Current specs (the bits that matter anyways):

Core 2 Quad Q9550 CPU (2.83 Ghz base, but i've got it running *very slightly* overclocked. I think it's running at around the 3 Ghz mark)
8GB DDR2 Corsair TwinX RAM
ABit IP35 Pro XE P35 mobo (Only 1 graphics card slot, so i'm not looking at any multi-card solution)
PowerColour HD 4870 512MB graphics card
CoolerMaster RealPower 850W modular PSU

I was thinking that something like an nVidia GTX 560 would be my best bet (I've always preferred nVidia, but at the time I built this ATI seemed to have a *much* better price/performance tradeoff), but i've got out of date with hardware since I built the system. I can't afford to upgrade the whole thing, and I was thinking a new graphics card would probably give me the most bang for my buck. If you think i'd be better saving up and getting a new mobo/CPU/RAM please tell me.

The bit that worries me is that my motherboard is only PCI-e 1.1 (At most. I can't find a definite value online, but the P35 Intel chipset certainly never supported PCI-e 2.0). The slot in the motherboard is also only PCI-e x16, not PCI-e x32 (does that only matter a lot if you're trying to do SLi or Crossfire? I don't know...). I don't want to go spending £200 on a shiny new card and then have it massively hamstrung to not much better than my current because it's in an older mobo. So that's my first question, is it worth me upgrading the graphics card to something in the £150-200 range (as of September/October 2011), or is that wasting my cash and I should not bother/get something cheaper?

The second question, of course, is what should I upgrade *to* if it is worthwhile? I'm in the UK, and generally buy from either www.scan.co.uk (most often) or, occasionally, www.ebuyer.com or www.overclockers.co.uk

I don't really have a vendor preference, I went with the PowerColour 4870 because it was the cheapest and it's served me well. I use a smaller quieter box as a media PC so I don't need HDMI out or similar, I just want two DVI connectors. I will say that I invested in a very large quiet CPU cooler, so if I could get a GPU that didn't sound like a plane taking off when I load a game it'd be nice, but it's not a deal breaker. I was thinking something like one of these:

Asus 1GB GeForce GTX 560 Ti DirectCU II TOP NVIDIA Graphics Card

Is it better to get the 560 Ti with more cores but a marginally lower clock speed, or should I get something with a marginally higher clock but fewer cores like this:

1GB Asus GeForce GTX 560 DirectCU TOP NVIDIA Graphics Card

Or would I in fact be better off sticking with AMD again and getting something like this:

XFX HD 6870 Dual Fan ATI - AMD Radeon Graphics Card - 1GB

I note that although there are 6870s of a similar price to the 560s I found that particular one is significantly cheaper (£140 as opposed to £180), so if we're looking at similar performance between that and the 560 i'm probably best off with that.

Ta in advance for any help, and sorry for the wall o' text

Performance/buck wise, the XFX 6870 is the best.
I do notice that ur board has PCi-E 1.1 and the Radeon card has PCI-E 2.1
but I would personally take the risk