Its a tricky question really, depending on what your psu is like, how much risk your willing to take and how tech-savvy you are or willing to be.
First off is the old multi card versus single card performance issue. As it stands not all games support crossfire well, but then again crossfire has improved a great deal and it is approaching being a practical solution, although its not quite there yet. Its a driver issue above all, and so you have to wait on improved catalyst drivers in the coming months (however as far as I know there are no crossfire benchmarks with the latest catalyst 7.11, so as to whether they are much better in this respect remains to be seen). Thats the risk part. Things look bright however in this respect as crossfire seems to be amd's strategy to reclaiming the performance crown from nvidia, what with quad crossfire on the horizon and the new radeon 3900: a duel gpu card, apparently ready hardware wise but waiting for the best possible drivers to make it work well in as many games as possible, which amd have given themselves until february to finalize. Thats a bit of a wait, but then again its a good idea to wait until you have 'killer' drivers before releasing the product that uses them, rather than releasing a product and ironing out driver issue as you go along (see: 2900xt ! )
Assuming all goes well or at least quite well, then you have to consider your psu. Both the 2900 series and the 8800gtx are power hungry, and you will need a quality psu over 500 watts for a single gtx and something around 650 watts for crossfired 2900 xt's. Whether a psu is 'up to the job' or not is not always a question of wattage, as amperage on the 12volt rails needs to be sufficient to. This usually means that a branded psu at a given wattage will be more capable than a cheaper generic psu at the same value, as the cheaper ones often cannot sustain performance at higher loads.
Lastly, I mentioned crossfire'd 2900 xts and not pros because it would appear that the pro is for all intents and purposes identical to the xt except with a different bios that sets the default clocks to 600mhz core, rather than the 740mhz of the xt. Im not entirely sure of this, but I believe the pro bios restricts the overclocking potential of the card. Every pro should be able to reach xt core and memory speeds easily on the standard cooler (the same cooler as the xt) and standard voltages but has trouble reaching the same max overclock as the xt. However if you flash the pro cards with an xt bios, it will use default xt clock speeds and be able to overclock to higher maximum speeds. Flashing the bios is the tech savvy part! it can go wrong!
note: despite what is said, 8 pin connectors for the 2900s are not needed to overclock them, but having them or using a 6pin to 8pin adaptor will make it easier.
Therefore crossfired xts or pros at xt speeds should run rings around a single gtx in games that support crossfire well. That list is not the largest right now, but should improve in the coming months. This crossfire will be power hungry, but perhaps your not overly concerned with that. A word of warning all the same! The final thing to consider is that both cards are old technology now, and so cheaper better replacements are either on the horizon in the gtx's case, or already here in the 2900's case. If you can pick up 2 2900pros cheaply or even second hand from, say, ebay, then with a bit of work and some luck on the driver side in the coming months you can put together a very powerful graphics setup, which should be able to max out any game except crysis (although crysis patches and newer drivers will supposedly improve crossfire and sli performance a great deal).
Note: although the 2900 series has now been replaced by the 3800s, in my personal opinion, so take it or leave it!, I dont believe that driver development for the older cards will come to a standstill now. Not only would that be very premature as the 3800s havent been out very long, and still are not available in huge volumes, but also the older and newer lines are very very similar architecturally. What applies to 3870 or 3850 crossfire should with a very small leap apply to 2900 crossfire too. Put it this way, I saw an article where a 2900 was crossfired with a 3870, with the 2900 as the primary card, and this mixing and matching actually worked. It didnt scale as well as to 3870's (this setup's performance was the thrust of the article) but the fact it functioned and scaled shows the similarities between the older and newer cards.
Well, hope that helps!