A lot depends on which VT-x features are supported by the processors you are concerned with. The initial implementations didn't provide measurable benefits and in most cases performance was actually degraded. However the recent H/W virtualization features related to Second Level Address Translation (SLAT) implemented by AMD as RVI\NPT with the Shanghai Opterons and Intel as EPT on Nehalem Xeons are certainly now delivering measurable benefits in many, if not most, use cases.
There's some interesting independant benchmarking in Johan de Gelas' "Real-world virtualization benchmarking: the best server CPUs compared" from May where he compares Sanghai and Nehalem systems with hardware virtualization features enabled and disabled although that is not the main intent of that article. It is independent however and de Gelas is always worth reading when he gets into the guts of CPU architectures and performance.
You can get some non-independent benchmarks from VMware that specifically detail the potential benefits of VT-x EPT on Nehalem CPU's in their whitepaper "Performance Evaluation of Intel EPT Hardware Assist". Your area of interest appears to be End User OS's rather than servers and Intel's EPT \ AMD's NPT can really make a difference there. The above paper covers a wide range of workload types but the two that may be of most interest to you were the Apache Compile (34-48% improvement) and Citrix XenApp (30%). VMware also published a similar paper last year for AMD systems with RVI\NPT.
Note that there can be downsides - the whitepapers demonstrate that some workloads do not benefit and may incur a small ( a few percent) degradation for workloads that don't benefit significantly from the improvements in page fault handling that EPT\RVI can deliver.
There is a very good detailed technical presentation from VMWorld 2007 that explains the progression and some of the reasons why the initial versions of VT-X\AMD-v did not deliver real world performance benefits and why the newer versions that support SLAT now do. VMWorld 1007 - "TA68 Performance Aspects of x86 Virtualization".
It's also important to make sure that whatever hypervisor you are planning to use is actually able to make use of all of the features - for example VMware vSphere\ESX4 or ESX 3.5 Update 4 is needed to support Intel's EPT, Update 3 (or newer) is needed to support AMD RVI\NPT.