First off, which Pentium4, specifically? Ideally the make/model of the system you saved.
At least some P4's pre-date PCIe ..... so your connectivity may limit your choices.
Assuming a newer P4 (04-06), you could probably pair with something up to about as strong as a 9800GT without too much issue.
The CPU may well hold back a 9800GT in some scenarios..... but given they sell for <10-15 GBP or USD, it wouldn't be too big a deal IMO.